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Turn things on or off if they’re too fast ... or too slow... etc! Deluxe Frequency Switch by John Clarke Switch devices on or off according to the frequency of just about any sensor signal up to 10kHz. So you can switch something on or off if a sensor signal frequency goes above or drops below a figure which you can easily set. Features • Energises a relay when a signal goes above a preset frequency and keeps it on until the signal drops below a second preset frequency • Adjustable hysteresis can be used instead of setting upper and lower frequencies • Switching frequency can be from 1Hz up to 10kHz. • Adjustable switching delay • Two sets of 5A changeover relay contacts • Easy pushbutton set-up • Can be set up on the bench or in situ • Threshold can be set using a signal generator or frequency meter (eg, DMM) • On-board signal frequency range indicators • Power, threshold and relay-on LED indicators T feathering blades on suitably and much more. here are many applications for equipped turbines. It is also much easier to set up a device of this type. Just some Of course, there are countless oth- than our previous Frequency Switch of the things we thought of “off er uses – you’re probably thinking of in June 2007 (siliconchip.com.au/ the top of our heads” include: • Cutting power (or fuel) to a motor others that suit your particular appli- Article/2261) and the main reason for cation. that is that it is based on a PIC16F88 if it exceeds a certain speed As long as it has, or can be fitted rather than the LM2917 frequency to • Switching a fan on at low vehicle speeds to provide improved cool- with a sensor, to provide a frequency voltage converter. (That first Frequenwhich varies with speed, temperature, cy Switch was quite tricky to set up!) ing. • Giving a warning to change gears flow etc, you can use our new Deluxe when the engine RPM is approach- Frequency Switch. It can do all of this Setting the two frequencies You need to set up two ing the tacho red line. frequencies, not one as you • Switching from long might have thought. to short intake run- Specifications Why do you need two ners at a particular en- Supply voltage: 10-16V frequencies? We need to set gine RPM to optimise Supply current: 20mA with relay off; 60mA with relay on two frequencies because if power delivery. Signal frequency range: 1Hz to 10kHz the signal from your cho• Switching off a pump sen sensor varies by even if a flow meter records Signal amplitude: >1.4V peak-to-peak a small amount at close to the water flow is out- Threshold setting resolution: 20Hz at 10kHz; 1Hz at 2.27kHz; 0.2Hz at the switching threshold, side a specific range. 1kHz; 0.002Hz at 100Hz. the relay would be con• Switching on an alarm Hysteresis: 0-50% stantly chattering on and if wind speed exceeds Switching delay: signal period plus 0-500ms off – not good at all. So a certain threshold. Signal frequency bands: <10Hz, 10-100Hz, 100Hz-1kHz, 1-10kHz we set an upper frequency • Applying a brake or 36 Silicon Chip Celebrating 30 Years siliconchip.com.au threshold above which the sensor signal must rise before the relay switches on. And then we set a lower frequency threshold below which the sensor signal must drop before the relay is switched off. You can set the two frequencies close together or far apart. Setting the frequencies is dead-easy and there are several methods for doing it. The first method is to feed in your wanted set frequency, say 500Hz, from an oscillator or other source to the sensor input and then press switch S2. Then feed in the wanted lower frequency, say 400Hz, and then press S1. The second method is arguably even easier. You just set one frequency, say 500Hz, and then use an on-board trimpot (VR1) to set the hysteresis. This will effectively set the lower frequency (down to a minimum of 250Hz in this example) and you can tweak it at the time of installation. If you don’t have an oscillator you could use the real signal that you intend controlling the unit with, so long as you can hold steady it at the required frequency/frequencies for long enough to press the switch(es). Alternatively, if that’s too difficult, you actually can get the microcontroller to generate the wanted frequencies. This second method is more involved than the first and we will describe the procedure later in this article. Detection time and delay You can also configure the unit with a switching delay which is adjusted with trimpot VR2 and can be set between zero and 500ms (ie, half a second). This ensures that if the signal frequency only momentarily crosses one of the thresholds, it will not cause the relay to switch. The input signal frequency must remain at or beyond the threshold for the entire delay time before any relay switching will occur. Each time the frequency crosses the threshold, the delay time starts again. Fig.1: This block diagram describes how the microcontroller measures frequency. If you prefer switching to happen immediately then set the response time to zero (ie, VR2 fully anticlockwise). LED indicators To help in the set-up and installation procedures, we have included indicator LEDs to show when an input signal is present and its frequency range: • LED2 lights for frequencies between 0.5Hz and 10Hz; • LED3 lights between 10Hz and 100Hz; • LED4 between 100Hz and 1kHz; • LED5 for frequencies between 1kHz and 10kHz and all four LEDs light if the frequency is above 10kHz. Other LEDs show when the set threshold frequency is reached and whether the relay is on or off. Relay options The relay is a double-pole changeover (double throw) type (ie, DPDT) which can switch one or two loads, each up to 5A/48V (8A if you use the specified relay from Altronics). You have the option to get the relay to switch on if the sensor signal rises above the threshold frequency (set by Here’s the complete frequency switch, ready to mount inside its case (it suits a UB3 jiffy box but could also be mounted inside the equipment it is controlling if there is room). siliconchip.com.au Celebrating 30 Years S2) and switch off if the sensor signal drops below the threshold set by S1. The alternative is to get the relay to switch on if the sensor drops below the lower threshold frequency (set by switch S1) and switch off if the sensor signal rises above the threshold frequency (set by switch S2). The second mode is activated by installing a link at JP1 on the PCB. Block diagram Fig.1 (above) shows how the Deluxe Frequency Switch monitors the signal frequency. The PIC16F88 micro’s internal clock is derived from a 20MHz crystal which is driven by an internal oscillator amplifier. The resulting 20MHz clock signal is divided by four to produce a 5MHz signal which drives an internal 16-bit timer, TIMER1. This comprises two 8-bit cascaded timers, TIMER1H and TIMER1L. We have implemented an 8-bit overflow counter (OVER) in the unit’s firmware. That extends TIMER1 out to 24 bits, so it rolls over every 3.355 seconds [or 224÷5,000,000]. This equates to an input frequency close to 0.3Hz. Hence, the unit is designed to handle signals from 1Hz and up. The input signal is fed to pin 6, which is also the Capture/Compare/ PWM (CCP) pin. The Capture module hardware in the micro is configured so that on each positive signal transition at this pin (ie, low-to-high), the values of TIMER1H and TIMER1L are copied into the CAPTURE1H and CAPTURE1L registers and an interrupt flag is set. This then trigMay 2018  37 Fig.2: the circuit is based around a PIC16F88-I/P, which measures the incoming frequency and energises the relay if the frequency is above or below certain values and whether JP1 is present or not. It also has pre-settable response times and hysteresis to prevent “chattering”. LEDs give you visual indication of the operation as well. gers an interrupt handler routine which copies the contents of the OVER register into the CAPTURE OVER variable. The timers and overflow counters are then reset to zero, ready to count until the next input positive going edge. The captured count represents the number of pulses from the 5MHz clock signal over the period between the two positive input signal edges. So for example, a 1Hz input signal will have a one-second period between each positive edge. The count value stored will therefore be 5,000,000 (5M). At 1kHz, the period between positive edges is 1ms and the captured value will be 5000. To calculate the frequency, all we need to do is to perform the calculation F(Hz) = 5,000,000 ÷ value. Or we can calculate the period as P(s) = value ÷ 5,000,000. But in reality, the micro just has to convert the upper and lower threshold settings to these same count units and then compare the counter values 38 Silicon Chip to those stored values, to determine whether either threshold has been crossed. On-board frequency generation Where the microcontroller produces an output frequency for you to measure during adjustment as per setup method on page 41), pin 6 (CCP1) is configured differently. Rather than being in Capture Mode, with pin 6 as an input, it is used in Compare Mode and pin 6 is an output. TIMER1 is still driven with the same 5MHz signal but the TIMER1L, TIMER1H and OVER registers are preloaded with values calculated from the frequency to be produced. Each time OVER register overflows, the pin 6 output toggles and new values are loaded into the TIMER1L, TIMER1H and OVER registers. Because pin 6 toggles each time the counters overflow, the output frequency would be half what you might expect based on the period value for Celebrating 30 Years the required frequency. So we need to divide the period by two to give two separate half periods. This means there will be an error whenever an odd period value is used, since dividing it by two will yield a remainder of one. To solve this, and avoid the inaccuracy, two different pre-load values are used. They are used alternately to load into the TIMER1L and TIMER1H registers. So the duty cycle will not quite be 50% but the frequency produced TP1 voltage Hysteresis (adjusted with VR1) when setting upper threshold 5V 3.75V 2.5V 1.25V 625mV 312.5mV 50% 43% 33% 20% 12% 6% Table 1: Hysteresis setting versus voltage at TP1. siliconchip.com.au will be accurate. The values from each of the separate period values are loaded into the TIMER1L, TIMER1H and OVER counters alternately. At the same time, the output at pin 6 is changed in level. For those interested, the values to pre-load into TIMER1L, TIMER1H and the OVER variable are calculated as 224 - (5,000,000 ÷ f (Hz)) ÷ 2, with the alternative value being one higher in cases where 5,000,000 ÷ f Hz is odd. Circuit description The full circuit shown in Fig.2 is based on microcontroller IC1, a PIC16F88. This monitors the input frequency, jumper state (JP1 and JP2), switch state (S1 and S2) and trimpot settings (VR1 and VR2). It also drives the frequency LEDs (LED2-LED5), threshold LED (LED6) and the relay coil (RLY1) and its associated LED (LED7) via NPN transistor Q2. Power is fed in via CON1 and the supply is nominally 12V DC. Diode D1 provides reverse polarity protection and its cathode connects directly to the positive terminal of the relay coil, applying a nominal 11.4V to it as well as to the 5V regulator, REG1 and it powers the rest of the circuit. A 10F electrolytic capacitor is used to filter the supply voltage and transients are clamped using a 16V zener diode (ZD1), with the peak current limited by the series 47Ω resistor. The supply is further filtered by another 10F capacitor and then REG1 reduces the 11.4V supply to 5V for IC1 and input conditioning transistor Q1. The power LED, LED1, is connected across the 5V supply with a 3.3kΩ series current-limiting resistor. The input signal is fed into CON2 and it’s AC-coupled via a 10F capacitor and 10kΩ resistor to the base of Q1. The 470pF capacitor filters any transients while diode D2 clamps the base voltage at -0.7V for negative excursions. Q1 inverts and amplifies the signal, suitable for the capture compare input (CCP1) at pin 6 of IC1. Frequency measurement modes When the micro is configured to generate frequencies for setting the upper and lower thresholds, the output signal appears at pin 6 and TP3. For this to work, there must be no input signal at CON2 and this means that Q1 is biased off and it will not load the siliconchip.com.au Fig.3: component layout for the Deluxe Frequency Switch with a matching photo below. We suggest using an IC socket for IC1 – and make sure when you place the connectors, their wiring access holes all point to the outside of the PCB. output signal from pin 6. 20MHz crystal oscillator X1 is connected to IC1, between its CLKO and CLKI pins, to allow for accurate and wide-ranging frequency measurements. The MCLR-bar reset input is tied to the 5V supply via a 10kΩ resistor to provide a power-on reset for the microcontroller. Internal pullup currents within IC1 hold the RB1 and RB2 inputs high when switches S1 and S2 are not pressed and similarly, are enabled for the RB5 and RB6 inputs which are connected to jumpers JP1 and JP2. These inputs are pulled low if a switch is pressed or jumper plug inserted and this can be sensed by IC1. Output pins RA0 (17), RB4 (10), RB7 (13) and RA1 (18) drive signal indicators LED2-LED5 via 3.3kΩ currentlimiting resistors at around 1mA each. Similarly, output RA4 (pin 3) drives the threshold LED, LED6. The RB3 output (pin 9) switches transistor Q2 on when it goes high. This transistor in turn switches on the relay. Diode D3 quenches back-EMF from the coil as Q2 is switched off. Celebrating 30 Years LED7 is also switched on when the relay is powered. It’s wired across the relay coil and uses a 10kΩ series resistor due to the higher voltage (11.4V). It provides the same current to LED7 as for the other LEDs. Trimpots VR1 and VR2 set the default hysteresis and delay time and both are connected across the 5V supply, with their wipers connected to analog inputs AN2 (pin 1) and AN3 (pin 2) respectively. The voltages at these pins are converted to digital values using IC1’s inbuilt 10-bit analogto-digital converter (ADC). The 100nF capacitors between each of these two pins and ground provide a low-impedance source for the ADC during conversions. Construction The Deluxe Frequency Switch is built on a double-sided PCB coded 05104181 and measuring 102 x 58.5mm. It will fit in a plastic utility box measuring 129 x 68 x 43mm. Follow the overlay diagram, Fig.3, when installing the parts. Fit the resistors first. The colour codes are shown May 2018  39 overleaf but we recommend that you use a digital multimeter (DMM) to check the values before soldering them. Diodes D1, D2, D3 and ZD1 are next and these need to be inserted with the correct polarity, with the striped end (cathode, k) oriented as shown in the overlay diagram. Diode D2 is the 1N4148 type while D1 and D3 are 1N4004. Parts list – Deluxe Frequency Switch 1 double-sided PCB coded 05104181, 102 x 58.5mm 1 DPDT 12V DC coil relay (RLY1) [Jaycar SY-4052 {5A}, Altronics S 4270A {8A}] 2 2-way screw terminals with 5.08mm pin spacing(CON1,CON2) 2 3-way screw terminals with 5.08mm pin spacing (CON3) 2 2-way pin headers with shorting blocks (JP1,JP2) 1 18-pin DIL IC socket (for IC1) 1 20MHz crystal (X1) 2 SPST PCB-mount tactile pushbutton switches (S1,S2) [Jaycar SP0600, Altronics S 1120] 2 PC stakes (TP GND,TP3) Semiconductors 1 PIC16F88-I/P microcontroller programmed with 0510418A.HEX (IC1) 1 LP2950ACZ-5.0 low dropout regulator (REG1) 1 BC547 100mA NPN transistor (Q1) 1 BC337 500mA NPN transistor (Q2) 1 16V 1W zener diode (1N4745) (ZD1) 2 1N4004 1A diodes (D1,D3) 1 1N4148 signal diode (D2) 7 3mm LEDs (LED1-LED7) Capacitors 1 100µF 25V PC electrolytic 2 10µF 16V PC electrolytic 1 10µF non-polarised (NP) PC electrolytic 4 100nF 63V/100V MKT polyester 1 1nF 63V/100V MKT polyester 1 470pF ceramic 2 27pF NP0/C0G ceramic Resistors & Potentiometers (all 1%, 0.25W) 1 1MΩ 1 100kΩ 4 10kΩ 6 3.3kΩ 1 1kΩ 1 47Ω 1 10kΩ vertical multi-turn trimpot, 3296W style (VR1) 1 10kΩ mini horizontal trimpot, 3386F style (VR2) 40 Silicon Chip We recommend using an IC socket for IC1. Take care with orientation when installing the socket and when inserting the IC. For the test points, we used two PC stakes, one for TP GND and the other for TP3. We left the remaining test points as bare pads so a multimeter probe can be inserted. Install the two 2-way pin headers for JP1 and JP2 and then follow with the capacitors. The electrolytic types must be fitted with the polarity shown (long lead to pad marked plus; the stripe indicates the negative side) and note that the 10F NP capacitor is non polarised and so can be installed either way around. Next, mount transistors Q1 and Q2 and also REG1. Take care not to mix them up as they come in identical packages. Trimpots VR1 and VR2 are next to be fitted. They may be marked as 103 instead of 10kΩ. Orient VR1 with the adjusting screw as shown. CON1 to CON3 can now be installed. CON1 and CON2 are 2-way types and CON3 comprises two 3-way screw connectors dovetailed together. Fit all connectors with the wire entry to the edge of the PCB. Finally, the LEDs and relay RLY1 can be installed. We placed the LEDs close to the PCB, but they can be mounted higher or mounted off the PCB if you wish, connected with flying leads. Although presented as a bare PCB, the unit fits in a UB3 Jiffy box. In this case, attach the PCB to the base of the box using spacers. First, mark out and drill 3mm holes for each of the corner mounting holes. You will also need to drill holes at each end of the box for cable glands. A gland at one end is used for the power and signal wires while a gland at the other end allows the relay contacts to be wired up as required. Set up You have several options for setting the unit up. You can set it up before installation using or an oscillator or the actual signal source (if it can be held steady enough) when you install it. 1) Oscillator method: Power the unit up with a 12V power supply wired to CON1. Connect the oscillator to CON2. Set the signal amplitude to 2V peakto-peak or 0.7V RMS. Set the oscillator to your desired upCelebrating 30 Years Using a tacho signal Say you are using the engine tacho signal to switch the relay if a certain engine RPM is exceeded – say 6000 RPM. If you have a 4-cylinder, 4-stroke engine, 6000 RPM = 100 revolutions per second. Since this type of engine fires two cylinders per crankshaft rotation, then the threshold should be set to 200Hz [100 x 2]. per threshold frequency (eg, 500Hz) and press S2. Then reduce the oscillator to set the lower threshold (eg, 400Hz) then press S1. That’s all there is to it. If you want to set a single threshold frequency (ie, the upper threshold) and use the hysteresis setting, fit a link to JP2. Then adjust trimpot VR1 for the required hysteresis (percentage) while you monitor the voltage at TP1. Then set the oscillator for the desired frequency and press S2. Alternatively, if you want to set a single threshold frequency at the lower threshold and use the hysteresis setting for the upper threshold, fit a link to JP2. Then adjust trimpot VR1 for the required hysteresis while you monitor the voltage at TP1. Then set the oscillator for the lower threshold frequency and press S1. Table 1 shows some the relationship between the voltage at TP1 and the percentage hysteresis. For example, if you set VR1 to give 1.25V at TP1, the hysteresis will be 20% and the resulting lower threshold frequency will be 20% lower than the frequency you set with switch S2. Note that you can also set the unit with only one threshold frequency and that will mean the relay will latch on when the signal goes above the threshold and will stay on until the power is turned off. To set just a single threshold frequency, set the oscillator to the desired frequency and then press S2. Then disconnect the signal from CON2 and wait until the signal LEDs all are off. Then press S1 to set the lower frequency to zero. No link is required at JP1 if you want the relay to switch on as the frequency rises above the threshold set by S2 (and turns off when the frequency drops below that set by S1). Alternatively, install JP1 if you want the relay to switch on as the frequency falls below the threshold set by S1 (and turn off when the frequency rises siliconchip.com.au above the threshold set by S2). lier) you need to adjust trimpot VR2. You can set the delay anywhere between zero and half a second. If you don’t want a delay set VR2 fully anti-clockwise. 2) Frequency meter method: The advantage of this approach is that you don’t need an oscillator but you will need a frequency meter or oscilloscope to measInstallation ure the frequency appearing at TP3. Connect the 10-16V To get into this DC power source between the mode, connect your +12V and GND inputs at CON1. frequency meter or For automotive installations, DMM between TP3 automotive-rated wire should and GND. be used and the +12V termiSwitch off power, nal needs to connect to the hold down both S1 switched side of the ignition. The PCB is designed to fit into a UB3 Jiffy box, as shown here – and S2 and then That way, the unit only opbut it could also be “built in” to equipment it is controlling. You switch on the powerates when the ignition is may also be able to source the 10-16V DC from that equipment – er. The micro then switched on and the vehicle as long as it isn’t turned off by the frequency switch! produces a 100Hz battery won’t go flat after long signal at TP3. Then remove JP1, use S1 & S2 to ob- periods of being parked. To adjust this default frequency to tain the lower threshold frequency, inThe easiest way to connect the GND obtain your desired upper threshold, sert JP1 again and press S1. terminal in a vehicle is to wire it to the press S1 and S2 until it reaches your Alternatively, if you just want to set chassis using a crimped eyelet secured target. S2 increases frequency, while the upper threshold frequency with S2 to a convenient screw terminal. S1 decreases frequency. and have the hysteresis setting made You may need to drill a separate hole Short presses of the switches will for the lower threshold as set by trim- in the chassis for this connection, or alter the frequency at a slow rate. For pot (VR2), then you must have a link utilise an existing earth connection. faster changes, hold the switch down fitted to JP2 before you start the proceWire CON2 to a suitable sensor. This and the rate will change to a faster rate dure. Similarly, you can set the lower can be the speedometer sensor, an ECU after two seconds. Continue to depress threshold with S1 and have the upper tachometer output, an injector or camthe switch for another two seconds and threshold set by the hysteresis percent- shaft position sensor and so on. If you the frequency will change at an even age value as set by VR2. haven’t already set the unit up, do so faster rate. Now turning off the power takes the as described above above. This allows you to run through the micro out of the mode whereby it proThe relay contacts are labelled Norentire frequency range in less than one duces an output frequency at TP3. It mally Open (NO), Normally Closed minute but still be able to do finer adthen reverts to normal operation, mon- (NC) and Common (COM). justments with brief switch presses. itoring the input frequency instead. To switch power to a load, wire one Having reached your target frequenThen fit a link to JP1 if you want the of its supply lines in series between cy, insert JP1. Then press S2 to set the relay to switch on as the frequency falls either the COM and NO terminals (so upper threshold frequency. Then rebelow the threshold set by S1 (and turns that it’s only powered when the relay move JP1. Then press S1 to reduce off when the frequency rises above that is energised) or COM and NC terminals the frequency to the lower threshold. set by S2). (so it’s switched off when the relay is Then re-insert JP1 and press S1 to set No link is required at JP1 if you want energised). the lower frequency threshold. the relay to switch on as the frequency Note that the relay contact current Note the two-step process to set each rises above the threshold set by S2 (and rating is 5A for the Jaycar relay and 8A frequency. In other words,with JP1 out, turns off when the frequency drops be- for the Altronics relay (see parts list). use S2 and S1 to adjust the frequency to low that set by S1). If a higher current is required, you the wanted value, insert JP1 and press If you want to configure the unit with can switch 12V DC to the coil of a largS2 to set the upper threshold. a switching delay (as described ear- er relay using RLY1. SC Resistor Colour Codes o o o o o o Qty. Value 1 1MΩ 1 100kΩ 4 10kΩ 6 3.3kΩ 1 1kΩ 1 47Ω siliconchip.com.au 4-Band Code (1%) brown black green brown brown black yellow brown brown black orange brown orange orange red brown brown black red brown yellow violet black brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown brown black black red brown orange orange black brown brown brown black black brown brown yellow violet black gold brown Celebrating 30 Years Small Capacitor Codes Qty. Value o o o o 4 1 1 1 F Code 100nF 0.1F 1nF 0.001F 470pF 27pF - EIA Code IEC Code 104 102 470 27 100n 1n 470p 27p May 2018  41