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Simple Waveform Generator This compact unit produces both square and triangle waves over the frequency range from 100Hz to 20kHz. Build it and use it to test audio amplifiers, filters, tone decoders and digi­tal circuits. By JOHN CLARKE A SIMPLE WAVEFORM generator is always useful to have on your workbench. It can be used as a signal source for all sorts of circuits, to test that they are operating correctly. A wave­form generator, even a simple unit such as that described here, is particularly useful for troubleshooting or when building circuits from scratch. This unit lets you select between triangle and square waves and you can vary the frequency output from about 100Hz to 20kHz using a single potentiometer. A second potentiome­ ter lets you vary the output level from 0-10.5V p-p for square waves, or from 0-4V p-p for triangle waves. All the parts, including the two pots, are mounted on a compact PC board, so that the assembly is really easy. But first, let’s find out how it works. Circuit details Fig.1 shows the circuit details. As can be seen, it’s based on the common 42  Silicon Chip 555 timer IC which is wired as an astable oscilla­tor. The timing components are connected to pins 6 & 2 of the IC, with the .01µF capacitor being alternately charged and discharged via VR1 (the frequency control) and its series 2.2kΩ resistor. The circuit works like this. Initially, when power is first applied, the .01µF timing capacitor is discharged and IC1’s pin 3 output is high. The .01µF capacitor now charges via the 2.2kΩ resistor and VR1 until it reaches twothirds the supply voltage (ie, 2/3Vcc). At this point, an internal comparator connected to pin 6 (the threshold input) of IC1 trips and this switches pin 3 low. The capacitor now discharges via VR1 and the series 2.2kΩ resistor until it reaches 1/3Vcc (the lower threshold). This point is detected by the trigger input (pin 2), which switches pin 3 high again. Thus, the cycle repeats indefinitely and pin 3 of IC1 alternately switches high and low while ever power is applied. Because it controls the charge/discharge times for the timing capacitor, VR1 effectively sets the frequency of oscillation. The nominal frequency (f) is given by the formula: f = 0.7/R1.C1 where R1 is timing resistance and C1 is the timing capacitance. In this case, R1 = VR1 + 2.2kΩ and C1 = .01µF. These values give a calculated frequency range of 200Hz to 45kHz. However, these figures don’t apply in practice because the output of IC1 does not go fully high. An 820Ω pullup resistor is used to pull pin 3 higher than it would otherwise go to give a more symmetrical waveform. However, the resulting frequency range of 100Hz to 20kHz is still less than calculated. The waveform at pin 3 is a nominal square wave, as shown in Figs.2 & 3. These show the square wave output at 19.7kHz and 122Hz, respectively. The duty cycle is not exactly 1:1 because of Fig.1: the circuit uses a 555 timer IC which is wired as an astable oscilla­tor. Transistor Q1 buffers the triangle output. pin 3 not going fully high but is near enough for our purposes. The waveform on pins 2 & 6 is triangle shaped since it represents the charging cycles of the timing capacitor. This output has a fairly high impedance, particularly at low frequen­ cies when VR1 is above 100kΩ. Transistor Q1 is used to buffer the triangle waveform. This transistor is wired as an emitter follower, which means that the signal on the emitter follows the signal applied to the base. Fig.4 shows the appearance of the triangle waveform when the frequency is about 5.4kHz. Switch S2 selects between the square wave at pin 3 of IC1 and the triangle waveform at the emitter of Q1. From there, the signal is fed to level control VR2 and then AC-coupled to the output termi­ nals via a 10µF capacitor. This capacitor ensures that the output signal has no DC component, while the associated 10kΩ resistor ensures that the output is always load­ed. Power for the circuit can be derived from virtually any supply capable of providing between 5V and 15V DC at about 20mA. The most convenient source for this would be a plugpack supply. Diode D1 provides reverse polarity connection protection, while LED1 is the power indicator. A 100µF capacitor decouples the supply to the circuit. Construction All the parts for the Waveform Generator are installed on a PC board coded 01307971. Fig.5 shows the wiring details. Begin the assembly by installing PC stakes at all external wiring points and at the connection points for the FEATURES Output waveform ................................................... Triangle or square wave Frequency range ..................................................100Hz to 20kHz nominal Square wave amplitude ....................................... 0-10.5V p-p (12V supply) Triangle wave output ................................................ 0-4V p-p (12V supply) Power supply ............................................................................... 5-15V DC Protection .......................................................... Reverse polarity protected Output impedance .............................................................................. <1kΩ Fig.2: this is the waveform that appears at pin 3 of IC1 when the output frequency is 19.69kHz. Fig.3: the waveform at pin 3 of IC1 when the output freq­uency is 122Hz. The duty cycle is not exactly 1:1. July 1997  43 Fig.4: this scope shot shows the triangle waveform at a frequency of about 5.4kHz. Note that this waveform was captured at the emitter of buffer transistor Q1. PARTS LIST 1 PC board, code 01307971, 60 x 105mm 1 500kΩ linear pot (VR1) 1 1kΩ linear pot (VR2) 2 knobs 2 DPDT slider switches (S1,S2) 10 PC stakes 1 20mm length of 0.8mm tinned copper wire Semiconductors 1 555 timer (IC1) 1 BC548 NPN transistor (Q1) 1 1N4004 1A diode (D1) 1 5mm red LED (LED1) Capacitors 1 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 .01µF MKT polyester Resistors (0.25W, 1%) 2 10kΩ 1 820Ω 2 2.2kΩ Fig.5: install the parts on the PC board as shown in this diagram. Make sure that all polarised parts are correctly orientated. rectly on the PC board, while the two pots are mounted by soldering them to their PC stakes. Cut the pot shafts to length before installing them and note that VR1 is a 500kΩ pot, while VR2 is a 1kΩ pot. Finally, complete the assembly by fitting rubber feet to the corners of the PC board. Testing Fig.6: this is the full-size etching pattern for the PC board. pots. Once this has been done, you can install D1, the resistors and the wire link. The IC and the transistor can go in next, followed by the capacitors and the LED. Take care to ensure that all polarised parts are correctly orientated. It’s 44  Silicon Chip quite easy to identify the LED leads, as the anode lead is the longer of the two. In addition, you will find a small flat area on the flange that runs around the bottom of the LED. This flange is always adjacent to the cathode lead. The two switches are mounted di- To test the unit, connect the output terminals to an audio amplifier, set the level control fully anticlockwise and apply power. If everything is working correctly, you should hear a tone in the amplifier’s loudspeaker when the level control is ad­vanced. Check that the frequency of this tone can be varied using the frequency control –this should range from 200Hz to beyond the limit of audibility. Alternatively, you can check that the unit is working properly by using an oscilloscope to monitor the output signal. If it doesn’t work correctly, check the board for solder bridges and missing solder joints. You should also check the supply rail to IC1 and to Q1’s collector. If the unit gives square waves but there is no output when triangle waves are selected, check the SC circuit around Q1.