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This tiny board is a voice-operated switch designed to fit into the tightest space. It uses a single IC & includes a SPDT 12VDC relay. It has almost no turn-on delay & a 3-second release time. By DARREN YATES S O WHAT IS A VOX? Well, VOX stands for voice-operated relay or switch. They’re most often used in “hands-free” communication such as amateur transceivers, mobile radios and some public address applications. The idea behind a VOX is that instead of the user pressing a button on the microphone to speak (ie, the press-to-talk switch), the sound of the voice is used to activate it instead. This leaves the user with an extra hand free to sit back with scones and a nice cup of tea. Our circuit uses one IC and a tiny SPDT (single-pole dou­ b le-throw) relay which can be used to switch on or off just about anything you like (but not 240V equipment). The relay we’ve used comes from Altronics (Cat.S-4140) and measures only 16 x 11 x 10mm – pretty tiny for a mechanical relay with a contact rating of 2A. In keeping with this, we’ve made the rest of the circuitry as small as possible so that you could install the VOX almost anywhere. It runs from a 12VDC supply and current drain is low, about 5 to 7mA quiescent and around 35mA when the relay is actu­ated. Circuit description The circuit for the MiniVOX is shown in Fig.1. As you can see, it is a “bare-bones” design involving an LM358 dual op amp (IC1), one transistor, the relay and a few other components. Voice signals are picked up by the electret microphone and fed to op amp IC1a. This is connected as a non-inverting amplifier with a gain of 151 or +43.6dB. The 100pF capacitor across the 150kΩ feedback resistor rolls off the high frequency response above 10kHz so that there is no chance of the circuit responding to spurious RF signals. The output of op amp IC1a at pin 1 feeds two diodes, D1 and D2, which Build the MiniVOX voice operated relay All the parts except for the electret microphone are mounted on a small PC board. Keep the microphone well away from the board, otherwise the relay noise will repeatedly trigger the VOX circuit. The circuit is suitable for switching low voltage equipment only (up to about 30V). function as a half-wave voltage doubler. These rectify the audio signal to produce a DC voltage across the 2.2µF capaci­tor which is directly proportional to the loudness of the sound fed to the microphone. This DC voltage is fed to the remaining op amp in the pack­age (IC1b) and this is connected as a comparator. The DC voltage from the rectifier is fed to pin 5 while a resistive voltage divider applies about +2V to pin 6. Once the DC voltage across the 2.2µF capacitor rises above the voltage at pin 6, pin 7 of IC1b pulls high, which turns on transistor Q1, a BC548 NPN type. This turns on the relay and lights up LED 1. Q1 remains on and the relay is actuated while ever the DC voltage at pin 5 is above the voltage at pin 6. Because of the high gain of op amp August 1994  31 D4 1N4004 10 10k 10k 3 8 IC1a 2 LM358 MIC 1 0.1 D3 1N4004 1k 2x1N914 D2 D1 100pF 6 1M K LED1 5 2.2 63VW  A 7 IC1b 12V PLUGPACK RLY1 10k B C Q1 BC548 E 4 2.2k 150k B 1k 2.2 25VW E C E B VIEWED FROM BELOW C A K MINIVOX VOICE OPERATED RELAY Fig.1: the circuit is based on dual op amp IC1. IC1a functions as a microphone preamplifier & this drives a diode charge pump based on D1 & D2. When the voltage across the 2.2µF capacitor on D2’s cathode exceeds a preset level, pin 7 of Schmitt trigger stage IC1b switches high & turns on Q1 & the relay. PARTS LIST 1 PC board, code 06109941, code 47 x 44mm 1 electret microphone insert 1 2A SPDT relay (Altronics Cat S-4140) Semiconductors 1 TL072, LM358 dual op amp (IC1) 1 BC548 NPN transistor (Q1) 2 1N914, 1N4148 small signal diodes (D1,D2) 2 1N4004 rectifier diodes (D3, D4) 1 red light emitting diode (LED1) Capacitors 1 10µF 25VW PC electrolytic 2 2.2µF 16VW PC electrolytic 1 100pF ceramic Resistors (1%, 0.25W) 1 1MΩ 1 2.2kΩ 1 150kΩ 2 1kΩ 3 10kΩ IC1a, together with the addi­ tional gain in the half-wave voltage doubler, the circuit has a very fast response to audio signals. On the other hand, the “release” time (the time taken for the relay to drop out) takes about three seconds, as determined by the time constant compris­ing the 2.2µF capacitor shunted by the 1MΩ resistor and the threshold voltage of IC1b, as set at pin 6. Diode D3 is connected across the coil of the relay to pro­tect the transistor when it switches off. If the diode was not there, the inductive kickback from the relay coil when the cur­rent is switched off could destroy the transistor. Power for the circuit can come from any 12VDC source; eg, car battery, DC plugpack, SLA battery – whatever you like. Diode D4 prevents reverse polarity connections from damaging the cir­cuit. Construction All of the components, including the LED and the relay are installed on a small PC board coded 06109941 and measuring 47 x 44mm. Before you begin any soldering, check the board thoroughly for any shorts or breaks in the copper tracks. These should be repaired with a small artwork knife or a touch of the soldering iron where appropriate. When you’re happy that everything appears to be OK, you can start construction by installing the wire link, followed by the resistors, diodes, capacitors, IC, the transistor, the LED and finally the relay. Note that to make the board as small as possi­ble, all of the resistors and diodes are mounted on their ends. Each component has a spacing of 0.2-inch or 5mm between its pins. Use the overlay wiring diagram to ensure that each compon­ent goes into the correct position. You will find that the circuit works best with the micro­phone connected to the circuit via a pair of flying leads about 50mm long. Don’t make them too long otherwise the leads may pick up hum. Because the relay switching itself makes noise, it’s quite easy for the circuit to “chatter” because of the relay sound being picked up by the mike. So keep the microphone away from the relay. RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  3 ❏  1 ❏  2 32  Silicon Chip Value 1MΩ 150kΩ 10kΩ 2.2kΩ 1kΩ 4-Band Code (1%) brown black green brown brown green yellow brown brown black orange brown red red red brown brown black red brown 5-Band Code (1%) brown black black yellow brown brown green black orange brown brown black black red brown red red black brown brown brown black black brown brown Another place for experimentation is in the threshold resistors. By adjusting the A N/C RELAY 10kΩ and 2.2kΩ resistors, you Q1 D3 can adjust the threshold or N/O A 1k more importantly, the on and 10k 10k 10uF off delay times. Having a higher 2.2k D2 threshold voltage will mean K IC1 that the circuit takes longer to 10k A LM358 D4 2.2uF 1 MIC switch on for some sounds and K 150k 12V will switch off sooner wherePLUGPACK D1 as a lower threshold voltage 100pF 2.2uF (achieved by reduc­ i ng the Fig.2 (left): some of the parts on the PC board are mounted “end-on” to save space, as 2.2kΩ resistor) would result shown on this wiring diagram. Fig.3 at right shows the full-size PC etching pattern. in a very quick on time and a longer release time. Don’t forget to include the PC stakes immediately and check your circuit You could also use this circuit as a as well. These will make it much easier against the overlay diagram for pos- very simple front door light whereby for you to solder the connecting leads sible errors. a sound triggers the relay to switch to the board. on a 12VDC light globe for say 30 secExperimentation onds. You could adapt the circuit to Installation This circuit provides plenty of do this by simply replacing the 2.2µF You can easily install the board possibilities for experimentation. By capacitor with a 10µF capacitor and in existing equipment wherever you using a 150kΩ feedback resistor with increasing the 1MΩ resistor to around can find enough room and a suitable IC1a, we have fixed the sensitivity of 3.3MΩ. The light globe connects to the 12VDC supply. the circuit to one which should suit relay outputs. Again, you can adjust the sen­sitivity When the unit is powered up, the most people. Alternatively, you could quiescent current should be around replace this feedback resistor with so that the circuit picks up sounds 5-7mA, increasing to around 35mA a 200kΩ logarithmic pot which will close to your front door and ignores with the relay in action. If it is sub- allow you to vary the sensitivity over cars passing in the street. Why not SC give it a try? stantially more than this, switch off a wide range. COM 0.1 1k 1M LED1 K August 1994  33