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Mini Projects #015 – by Tim Blythman SILICON CHIP Analog Servo Gauge A gauge with a needle is often the simplest way of communicating a reading. This project lets you convert an analog voltage to a gauge readout. Because it uses a servo motor, you can make it really big! › Only needs a 5V DC supply › Span and offset adjustment trimpots › Converts a 0-5V signal into a PWM signal to drive a servo motor › Uses just one comparator IC and a voltage regulator, plus some passives. T his project displays a voltage from 0-5V using a moving needle. While simple analog voltmeter movements can do this, they are delicate, somewhat expensive, and limited in size due to the movement’s strength. Our servo motor allows a much larger pointer to be used. That means some extra circuitry is needed, but our circuit uses just a few inexpensive parts and can be built on a prototyping board in under an hour. The servo we are using (intended for use in remote-controlled [RC] vehicles and such) comes with mounting screws and plastic arms (‘horns’), so it is easy to attach it to a dial to suit your application. Making a suitable needle that can be affixed to the horn is also straightforward. This sounds like the perfect application for a small microcontroller board like an Arduino; it would need just a single analog input and one digital output pin. However, servo motors similar to those we are using were invented before microcontrollers. So we can drive the servo motor using some old-fashioned analog electronics. You can see a video of the Servo Gauge working at siliconchip.au/ Videos/Analog+Servo+Gauge How an RC servo works The term ‘servo motor’ has a broader scope than just the type we are using in this project. In general, any motor that uses a feedback system to attain accurate positioning can be considered a servo motor. Specifically, we use a standard three-wire servo, as used for radio control (RC) and robotics. Apart from 5V power and ground, this type of servo has a digital input that accepts a pulse train. The pulses are sent around 50 times per second; the exact rate is unimportant but the pulse width is. Pulses around 1-2ms are commonly used. These servos have a shaft connected to a potentiometer. When the servo receives a pulse, it generates its own pulse, the length of which depends on the potentiometer position. By comparing the pulse lengths, the Some components are packed quite closely around IC1 (as can be seen in the lead photo), but you should be able to squeeze them all in with some care. Note the blue wire and two bridged pads on the back of the PCB (circled in white). Australia's electronics magazine siliconchip.com.au servo knows whether it needs to turn clockwise, anti-clockwise or stay still (when it has reached the desired position). Bob Young’s article in the March 1991 issue explains this in more depth (siliconchip.au/Article/7102). Unsurprisingly, modern servos contain a microcontroller, but they are still compatible with the same protocol that dates back to the 1960s. So we can easily interface with modern servo motors using electronics of a similar age. Circuit details Our circuit (shown in Fig.1) consists of several simple sections with distinct purposes. The section around REG1 at upper right generates a stable 3.3V for the rest of the circuit from the 5V DC input. Since the servo can draw relatively high current pulses that might affect the 5V rail, this is necessary to ensure the rest of the circuit does not change its behaviour. We are using an LM2936-3.3 regulator with the two capacitors it requires at its input and output. The other two parts of the circuit each use half of an LM393 dual comparator IC. As the name suggests, this IC compares the voltages of its two input pins. If the + (non-inverting) input (pin 3 or 5) is higher than the – (inverting) input (pin 2 or 6), the corresponding output pin (pin 1 or 7) is not driven. If the inverting input is higher than the non-inverting input, the output is pulled to ground (0V). This is known as an ‘open collector’ or ‘open drain’ since it is usually implemented with a transistor where the collector (or drain) is only connected to the output pin. The circuit around IC1a is a sawtooth waveform generator. Initially, the 2.2µF capacitor is discharged and the V_SAW level (and thus pin 2) is at 0V. Around 2.2V is on pin 3, so the output at pin 1 is not driven and thus pulled up by the 1kW resistor. The 2.2µF capacitor charges up via the 1kW and 4.7kW resistors until it reaches 2.2V, at which point the comparator output goes low. This causes the capacitor to start discharging via the 4.7kW resistor, into the comparator’s low output pin. At the same time, the voltage at pin 3 goes to around 1V. When the capacitor (V_SAW) reaches 1V, the comparator output changes again and the cycle siliconchip.com.au Fig.1: 3.3V regulator REG1 ensures variations in the supply voltage don’t affect the pulse timing. One half of the comparator (IC1a) provides a sawtooth waveform, while the other half (IC1b) uses that to generate pulses suitable for driving the servo motor. 5.0 4.0 3.0 2.0 1.0 0.0 -1.0V -20.0ms 0.0 20.0 40.0 60.0 80.0 100.0 Scope 1: the blue trace is V_SAW (pin 5 of IC1b), green is pin 6 of IC1b, yellow/ brown is the servo control signal from pin 7 of IC1b and red is output pin 1 of oscillator IC1a. 5.0 4.0 3.0 2.0 1.0 0.0 -1.0V -20.0ms 0.0 20.0 40.0 60.0 80.0 100.0 Scope 2: this is the same as Scope 1 except that the green trace voltage has changed slightly due to varying the control signal voltage, resulting in a change in the pulse width of the yellow/brown trace that goes to the servo motor. Australia's electronics magazine October 2024  69 We created this simple design, printed it out and glued it to some cardboard to suit a 5V scale over about 90°. The needle is simply a piece of dark-coloured cardboard glued to one of the servo horns. All the necessary screws should come bundled with the motor. Watch the polarity of the electrolytic capacitors; their negative leads all connect to the ground rail. continues around 40 times per second. Scope 1 shows the V_SAW voltage (the blue trace) and the pin 2 comparator non-inverting input (red trace). The arrangement of resistors and potentiometers connected to the second comparator translates the input voltage (from the Control input) to a voltage suitable for feeding to comparator IC1b. The modified voltage fed into IC1’s pin 6 is the green trace in Scope 1, while the output to drive the servo (from pin 7) is the yellow trace. The stack comprising the 4.7kW resistor, 1kW potentiometer and 10kW resistor puts the green trace just below 2.2V, near V_SAW’s peak, so we get the brief pulses needed to drive the servo. The 10kW potentiometer allows us to set how much of the Control input signal is passed on to the rest of the circuitry, while the 100kW resistor ensures that the Control input only has a small effect on the green trace level. The 2.2µF capacitor in this part of the circuit ensures that the control voltage doesn’t change too rapidly. If the voltage here jumped around too fast, it could cause glitches that would make the motor behave erratically or even damage it. Scope 1 was captured with the control input at 0V, while Scope 2 has the control input at 5V; otherwise, the circumstances are identical. You can see that the green trace has lifted slightly, causing the pulse width to nearly halve. That gives the required 1-2ms pulse range to control the servo over a roughly 90° range of rotation. Construction The first step is to build the circuit, which can be done on a small prototyping board with a similar layout to a breadboard (except that the power rails are down the middle). You don’t have to follow our layout strictly, but we know it works, so you might find it easier to match it. Check our photos and the layout diagram, Fig.2, while you solder the components to the board and add the Fig.2: here is how we have laid out the components on a prototyping board. Note that there is a single wire link under the IC, between pins 2 and 5, shown in cyan. The ground and 5V supplies for the IC are also connected by bridging pins 4 and 8 to their power rails with solder blobs. 70 Silicon Chip wires. While most features are visible from the top of the board, a wire and a couple of solder links are on the underside (see the photo on the opening spread). Start by fitting the IC socket; this will make it easier to run some tests with the IC out of circuit. Note the direction of the notch (to the left). Install the parts as shown, paying attention to the orientation of the electrolytic capacitors. After fitting all the components except IC1, add the wires shown. Three are on the copper side of the board, under the IC1 socket. In addition to those, there are two dark grey ground wires, two orange 3.3V power wires and one cyan/blue signal wire; don’t forget to add any of them. After that, connect a 5V DC power supply and run some tests. We used cut-off jumper wires so that we could plug into an Arduino board for power but you might have a different idea. Apply 5V and check that you get 3.3V at pin 1 of the regulator (towards Fig.3: use this guide to help cut a hole to suit the servo motor. It can be copied (or downloaded and printed) for use as a template. Australia's electronics magazine siliconchip.com.au the bottom in Fig.2); you should be able to measure different voltages of around 2-3V at pins 1, 2 and 3 of IC1’s socket. Pin 6 of the IC socket should be about 2.0-2.2V. Disconnect the power and plug IC1 into its socket, being careful not to fold up any of the pins under its body. Power on the circuit and connect the servo motor to the three-way header. It will probably run to one of its end stops and stall. Adjust the 1kW trimpot so that it is near the middle of its travel. It should work backwards; that is, turning the trimpot clockwise will cause the servo to turn anti-clockwise. If it is not responding, disconnect the power to avoid damaging the servo’s mechanism and motor, then check your wiring. If all is well, connect a jumper wire from the signal input (where the blue wire is shown in Fig.2) to 5V. You should then be able to move the servo by adjusting the 10kW trimpot. Again, be careful not to allow the servo to run against its end stops excessively. Parts List – Servo Gauge (JMP015) 1 micro servo motor [Jaycar YM2758] 1 25-row prototyping board [Jaycar HP9570] 1 8-pin IC socket [Jaycar PI6500] 1 3-way header, 2.54mm pitch [cut from Jaycar HM3212] 2 2-way headers, 2.54mm pitch [cut from Jaycar HM3212] 1 10kW side-adjust mini trimpot [Jaycar RT4016] 1 1kW side-adjust mini trimpot [Jaycar RT4010] 1 10cm length of insulated wire 1 5V power supply (see text) 1 gauge face and needle to suit (see photos) Semiconductors 1 LM393 dual comparator, DIP-8 (IC1) [Jaycar ZL3393] 1 LM2936-3.3 3.3V LDO voltage regulator, TO-92 (REG1) [Jaycar ZV1650] Capacitors 1 10μF 16V radial electrolytic [Jaycar RE6066] 2 2.2μF 63V radial electrolytic [Jaycar RE6042] 1 100nF 50V multi-layer ceramic or MKT [Jaycar RM7125] Resistors (all ¼W or ½W 1% axial) 1 100kW [Jaycar RR0620] 1 10kW [Jaycar RR0596] 2 4.7kW [Jaycar RR0588] 2 2.2kW [Jaycar RR0580] 3 1kW [Jaycar RR0572] Turning it into a gauge You have a bit of flexibility in choosing your gauge face and pointer. The servo should be supplied with screws and plastic horns for mounting. The photos show the basic gauge we created, with a printed piece of paper glued to some cardboard, to show readings from 0V to 5V. The servo will have a usable span of just over 180°, but we’ve gone for a more traditional analog gauge range of about 90°. Fig.3 shows the dimensions of the holes for the servo, which should help you to cut out your gauge face to suit. There is one rectangular cutout to make plus two small holes for self-tapping screws to retain the servo motor. The grey-shaded circle shows the servo shaft, which serves as the pivot point for the Gauge. Fig.4 shows the image we printed to make the gauge face; it is available as a PDF download from siliconchip. au/Shop/11/488 For the needle, we screwed one of the horns to the servo shaft, then glued a pointer to it so that it pointed at the 0V marker. Using it To calibrate the Gauge once the glue has set, power the circuit and connect the voltage input to 0V (eg, ground on the protoboard). Then adjust the 1kW siliconchip.com.au Fig.4: the gauge panel artwork we created shown at 90% of actual size. You can download it as a PDF from siliconchip.au/Shop/11/488 trimpot until it points at the 0V point on the gauge. Next, connect the input to 5V (or whatever your maximum will be). The 3.3V rail is another well-defined and accurate level. Adjust the 10kW trimpot so that it points accurately for the higher input. Australia's electronics magazine The two inputs interact slightly, so switch back and forth between them a couple of times to make minor adjustments until the Gauge is operating accurately. Remember that the 10kW trimpot will slightly load the source of the control voltage. SC October 2024  71