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Triggers devices on and off with sound Build a VOX By JOHN CLARKE Traditionally, VOX circuits toggle a transmitter on as you speak into a microphone and off again when there is silence. But VOX circuits can be used anywhere you want to turn something on when a sound occurs or you speak into a microphone. You could use it turn on a light, an amplifier or maybe even unlock a door. VOX stands for Voice Operated eXchange and it is also the Latin word for ‘Voice’. A VOX circuit switches on a relay whenever a signal reaches a set threshold. The relay switches off once the signal level drops below the threshold and after a short delay. They are used in communications, public address systems, surveillance, security and general purpose electronics. For communications, a VOX switches a transceiver from receive to transmit whenever the person speaks into the microphone. This frees the operator for other tasks as a separate switch is not needed to talk. Many intercoms and public address systems are also automated in a similar way. 82  Silicon Chip A VOX circuit can be used to mute any sound until it reaches a set level. That way a public address system will ignore background noise and remain quiet, until someone intentionally speaks into a microphone. For security and surveillance, a recorder can be switched on whenever a noise is sensed by a microphone. But it doesn’t have to be a microphone which causes the VOX action. For general-purpose use, any audio signal can used to switch the relay. Our design In line with the above comments, our VOX design has two inputs, both of which will accept the same types of audio input. First is a stereo 3.5mm jack socket which will handle both mono and stereo signals, while the second input is for mono inputs only and is via screw terminals. You can connect an electret or dynamic microphone. Electret microphones require a bias voltage which can be selected with a jumper link (LK1). For stereo signals connected via the 3.5mm socket, a jumper link provides mixing of the left and right channels into a mono signal. Signal sensitivity can be adjusted to cover a wide range from microphone levels up to line levels of 2V RMS. With sufficient signal, the relay switches on and remains on until the signal level drops to below a threshold level. An adjustable delay sets the time taken for the relay to switch off once this threshold is reached. The relay has two sets of changeover contacts which will suit a variety of siliconchip.com.au Features • • • • • • • • • 12V operation Electret or dynamic microphone or line input 3.5mm jack socket or screw terminal inputs Mono or stereo signal Adjustable sensitivity Adjustable delay Hysteresis prevents relay chattering at threshold DPDT relay Power and relay LED indication switching applications. LEDs are included for visual indication of power and of relay switching. Because of the wide variety of possible uses for a VOX, our module is simply presented as a PCB which you can install to suit your application. Or if you wish, it can be fitted into a plastic “UB3” case measuring 130 x 68 x 44mm. As you can see from the features at left, our new VOX is quite a versatile beast! It can be used in practically any application which requires triggering from a sound source – and that sound source can itself be just about anything! a supply that is decoupled from the 11.4V supply via a 1k resistor and a 100F capacitor. This decoupling prevents supply variations entering the input to the amplifier to cause false triggering. If the electret microphone is connected via the stereo jack socket input, the electret is connected between the ground terminal (sleeve) and the tip of a mono jack plug. Again, link LK1 is inserted for electret power. If an electret is not used and signal is applied via the jack socket or screw terminals, the link (LK1) is left disconnected. Stereo signals can be connected via the stereo jack socket and the signal is mixed down to mono using 10k resistors for each channel. This stereo mixing occurs when link LK2 is inserted. Dynamic microphones do not require bias current; in fact they should not be connected to a circuit providing electret bias, hence the reason for LK1. A 100nF capacitor couples the mono signal to op amp IC1a. Its noninverting input, pin 3, is biased from the decoupled supply via two 100k resistors. This sets the ampli-fier output to swing symmetrically about Circuit details a nominal half-supply voltage. The The VOX comprises a dual op amp half supply will vary from about 5.3V (IC1) that functions as a signal amplito about 5.6V, depending on whether fier and threshold switch. The relay or not an electret microphone is conis driven from the second op amp via nected. a transistor. Diodes D1 and D2 are included to Input signals come in via the 3.5mm clamp any signal to +0.6V above the jack socket (CON1) or via a 2-way decoupled supply and -0.6V (ie, below screw terminal block (CON2). For the the 0V rail). They protect the IC1 input screw terminal input, if an excessive signal is one terminal is conapplied. nected to ground while IC1a is connected as a Power supply:.................... 12VDC at 50mA the other is applied to non-inverting amplifier Trigger sensitivity:............. Adjustable from 2mV (microphone) to 2V (line) the amplifier stage via with a gain of 2 when Maximum signal input: ..... 50V rms a 10k resistor. VR1 is set to zero ohms Signal frequency range: ... 16Hz to >600Hz When an electret and a gain of about 1000 Attack time:........................ 10 cycles with signal at threshold voltage microphone is used, when VR1 is set to its (faster attack if signal is above threshold) bias current is selectmaximum. The actual Hysteresis:......................... 0.44V at the 2.06V threshold ed when link LK1 is gain when VR1 is set to a Delay time:......................... Adjustable from 100ms to 10s closed. The 10k bias high value is dependent Electret bias current:......... ~320 A resistor is connected to upon the signal frequency Relay contacts (DPDT):...... 5A (maximum of 50V recommended) siliconchip.com.au July 2011  83 Specifications 1k 10k LK1 LK2 CON1 100 F 16V K 100k 100nF 3 8 IC1a 2 100 1 D4 1N4148 10 F 47k K A D3 1N4148 4 100 F 1k A SENSITIVITY 1k 7 IC1b VR2 100k K 1k VR1 1M 5 6 47pF LK1: ELECTRET BIAS LK2: STEREO 1M K NP K 100k 10k  POWER 2 x 10k 100 F 16V A LED1 A ALTERNATIVE ELECTRET OR SIGNAL INPUT 4.7k 100 F 16V A D2 1N4148 + 10 F 16V 100nF D1 1N4148 3.5mm JACK SOCKET CON2 +11.4V 2.2k 100nF IC1: LM358 10 F AC SIGNAL TO DC CONVERTER (RECTIFIER) AMPLIFIER SC 2011 VOICE ACTIVATED RELAY DELAY SCHMITT TRIGGER 1N4148 1N4004 A A K K 84  Silicon Chip D3 D4 4148 4148 47pF 4.7k 47k 10k 1k 4148 1k 100k 4.7k A K 100 F CON5 NO COM 1M VR2 100k 100 F NO COM NC X OV 4004 Q1 D6 10k 10k CON4 1k 2.2k 100nF D2 10 F 100nF 1k 4148 100k 10k 10k CON1 NC RELAY1 11170210 VR1 1M IC1 LM358 100nF 10 F LK2 SIG IN 100 F 10k GND LED2 NP 100 F LK1 0V 4004 +12V CON3 K 10 F 100 D5 A LED1 CON2 and the open- loop gain of the LM358 op amp. The 47pF capacitor is included to provide a steep roll-off at high frequencies, to ensure IC1 does not oscillate. However, it is the open-loop gain of the amplifier that sets the bandwidth. For example at a gain setting of 100 (when VR1 is 99k), the roll-off caused by the 47pF capacitor is about 34kHz. Roll-off due to the open-loop gain is at around 6kHz. With VR1 set for a gain of 1000, the 47pF rolls off frequencies above about 3.4kHz. But the open-loop gain begins to roll off beyond about 600Hz. Low frequency rolloff is set at about 16Hz. This is due to the 1k resistor and 10F capacitor connected in series to the inverting input. The output signal from op amp IC1a is fed to a rectifier involving diodes D3 and D4, to convert the AC signal to a DC voltage. As pin 1 swings above its resting position of 5.7V, the 10F capacitor discharges via diode D4 into the 100F capacitor at D4’s cathode. When pin 1 swings below 5.7V, the 10F capacitor discharges via D3. The 100F capacitor then charges with repetitive pulses provided by the 10F capacitor. Op amp IC1b is connected as a Schmitt trigger comparator, with the inverting input at pin 6 tied to a voltage divider comprising a 10k and 2.2k resistor across the 11.4V supply. Pin D1 Fig.1: complete circuit diagram of the VOX, or Voice Activated Relay. It’s all based on one IC, an LM358, which performs the dual function of signal amplifier and comparator/schmitt trigger. A handful of other components complete the circuit. Fig.2: everything mounts on the one PCB, shown here in both diagram and photo form. The only thing “missing” from the PCB is the microphone which must be mounted off the board, as it will “hear” the relay pulling in and releasing and more than likely trigger in error. It can be mounted on a short pair of wires if you wish, or as long away as necessary using a shielded microphone cable. siliconchip.com.au D5 1N4004 K A +12V A 0V RELAY  LED2 K RLY1 K D6 1N4004 4.7k CON5 A NC COM NO 10k B CON4 C Q1 BC337 NC COM NO E 10k RELAY DRIVER BC337 LEDS B K A E C 6 sits at about 2.06V and is bypassed with a 100nF capacitor. IC1b’s non-inverting input, pin 5, monitors the voltage across the 100F capacitor via a 47k resistor. When the 100F capacitor voltage is below pin 6, IC1b’s output at pin 7 is low; close to 0V. When the capacitor voltage rises above pin 6, pin 7 will go high to about +10V. So provided the AC signal fed to rectifier is enough to produce more than 2V across the 100F capacitor, pin 7 of IC1b will go high and this will turn on transistor Q1 and the associated relay. Now one of the problems with a trigger circuit like IC1b is that it will not switch cleanly from high to low since a very slight change in the voltage across the 100F capacitor could mean that it switches back and forth very rapidly. This would have the result that the relay would chatter, ie, also switch on and off very rapidly. We fix this by adding hysteresis to the circuit, by including the 1M resistor between pin 5 and 7. What now happens is that when the output switches high, it also pulls pin 5 slightly higher, 0.35V higher than the 100F capacitor voltage. This means that the capacitor has to discharge by this amount before the IC1b will go low again. This stops the relay chatter. The 100F capacitor is continually discharged via VR2 and the 1k resistor. So if signal from IC1a is not siliconchip.com.au Parts List – VOX CON3 1 PCB coded 01207111, 106 x 61mm 1 DPDT 12V relay, 5A contacts (Jaycar SY-4052, Altronics S4190C) (RLY1) 1 3.5mm stereo socket PCB-mount (Jaycar PS-0133, Altronics P0092)) (CON1) 2 2-way PCB-mount screw terminals with 5.08mm pin spacing (CON2,CON3) 2 3-way PCB-mount screw terminals with 5.08mm pin spacing (CON4,CON5) 1 electret microphone insert (MIC1) (if required – see text) 11M horizontal mount trimpot (Code 105) (VR1) 1100k horizontal mount trimpot (Code 104) (VR2) 2 2-way pin headers with 2.54mm pin spacing (LK1,LK2) 2 2.54mm jumper shunts 4 M3 tapped spacers (optional) 4 M3 x 6mm screws (optional) 1 length of hookup wire or single cored shielded cable Semiconductors 1 LM358N dual op amp (IC1) 1 BC337 NPN transistor (Q1) 4 1N4148 switching diode (D1-D4) 2 1N4004 1A diodes (D5,D6) 2 3mm red LEDs, 1 red and 1 green (LED1,LED2) Capacitors 3 100F 16V electrolytic 1 10F Non Polarised (NP) electrolytic 2 10F 16V electrolytic Codes: 3 100nF MKT polyester 1 47pF ceramic Resistors (0.25W 1%) 4-Band Code (1%) 1 1MΩ brown black green brown 2 100kΩ brown black yellow brown 1 47kΩ yellow purple orange brown 6 10kΩ brown black orange brown 2 4.7kΩ yellow purple red brown 1 2.2kΩ red red red brown 4 1kΩ brown black red brown 1 100Ω brown black brown brown continuously replenishing the 100F capacitor, the voltage will drop in level. VR2 sets the delay period from when IC1b is triggered high to when its output goes low in the absence of signal from IC1a. The VOX runs from a 12V supply and diode D5 is included for reverse polarity protection. LED1 indicates when power is present. Construction The VOX is assembled on a PCB coded 01207111 and measuring 106 x 61mm. All of the components are mounted on the PCB, apart from the microphone which must not be – it needs to be off the board so that it does not attempt to retrigger the F Value IEC Code EIA Code 0.1F 100n 104 NA 47p 47 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow purple black red brown brown black black red brown yellow purple black brown brown red red black brown brown brown black black brown brown brown black black black brown circuit whenever it “hears” the relay switch off. The PCB is sized to clip into the integral side slots of a UB3 utility box measuring 130 x 68 x 44mm. If you are using this box, make sure the left edge of the PCB is shaped to the correct outline so it fits into the box, clearing the internal corner pillars. That way the 3.5mm socket can pass through the end of the box. It can be filed to shape if necessary, using the PCB outline shape as a guide. Begin construction by checking the PCB for breaks in tracks or shorts between tracks or pads. Repair if necessary. Check hole sizes for the components and for the corner mounting holes. July 2011  85 Another view of the VOX PCB showing in detail the input and power sockets. It can be driven from an electret microphone (as shown here), a dynamic microphone (with the bias link LK1 left open) or indeed from virtually any audio source from 2mV (microphone level) right up to 2V (higher than most line levels). The sensitivity pot (closest to the input sockets) can be adjusted to cater for this range. The other pot (closest to the relay) adjusts the length of time the relay stays closed once it is triggered. Assembly can begin by the inserting the resistors. When doing this, use the colour codes in the parts list to help in reading their values. A digital multimeter can also be used to measure each value. Next come the diodes, remembering these must be mounted with the orientation as shown. There are two types of diodes; the smaller 1N4148s are D1-D4 while the larger 1N4004 types are D5 and D6. IC1 can be soldered directly into the PCB (or you can use a DIP8 socket if you wish). When installing the IC (and socket), take care to orient them correctly. Orientation is with the notch positioned as shown. Capacitors can be mounted next. The electrolytics must be oriented with the shown polarity except for the NP (non-polarised) type that can mount either way. Mount the transistors and trimpots VR1 and VR2. VR1 is the 1M trimpot and could be marked with its value or with the coding 105. The 100k VR2 could be marked as 104. LED1 and LED2 are mounted about 5mm above the PCB. The anode is the longer lead and is placed in the uppermost hole. The 2-way pin headers for LK1 and LK2 can be mounted now, followed by the 3.5mm socket, the relay and the screw terminals. CON1 and CON2 are 2-way terminals that are first attached by sliding the dovetail sections of each together. Similarly for the CON3 and CON4 terminals, these are slid together before being mounted on the PCB. Make sure the wire entry side face the outside of the PCB. 86  Silicon Chip We mounted the PCB on four 6mm long tapped spacers, held in place with M3 x 6mm screws but this is entirely up to you and your application. If using an electret microphone, this should be mounted so that it does not touch the PCB and connected via multi-strand hookup wire for short (less than 30mm) leads or using single core shielded cable for longer runs. The shield wire connects to the GND terminal (for the 3.5mm jack plug, the GND is the sleeve). Signal connects to the second screw terminal for the screw terminal input or the tip connection of the 3.5mm jack plug. For a signal input other than a microphone, apply the signal to either the screw terminals or via a 3.5mm jack plug. One channel connects to the tip terminal and the other channel to the ring terminal. Link selection depends on whether you are using an electret or dynamic microphone or a mono or stereo signal connection. LK1 should be linked only when the electret microphone is used and removed for a dynamic mic. LK2 should have a jumper link for a stereo signal. You wouldn’t normally have both LK1 and LK2 in position at once but there are stereo electret microphones around so it is possible (though why you’d want to use one in this application is a bit beyond us!). Apply 12V power and adjust VR1 so that the relay triggers at the required signal level. Similarly, adjust VR2 so that the relay switches off after the desired time period. The delay should be as short as possible but not so short that it drops out while speaking. If the Voice Activated Relay does not work, first check your soldering to make sure there are no dry joints, solder bridges or dags, etc. If the visual inspection looks OK, check voltages on the circuit. There should be about 11.4V between pins 4 and 8 of IC1. Pin 3 of IC1 should be around 5.7V to 5.3V. Pin 6 of IC1b should be at about 2V. Incorrect voltages may be because of incorrect resistor values or a short or open circuit connection. Check that LED 1 lights. Output of IC2 at pin 7 should be near 0V when no signal is applied (or when no sound is detected by the microphone). With sufficient signal applied, the pin 7 output should go to around 10V, the relay should switch on and LED2 should light. The relay should switch off after the preset time period when there is no signal. 9V operation? We know we will be asked the question! Some constructors may wish to use the VOX as a stand-alone device – so we’ll answer it already! No, operation from 9V would be quite unreliable, especially if the battery is a bit flat. And the 50mA current draw would put the battery in that state pretty quickly! Most of the circuit would be fine at 9V but the 12V relay would not be at all happy (if indeed it worked at all). Substituting a 5V relay may be an option, with a resistor in series with the coil but it may not be possible to get one which fits the PCB without modification. SC siliconchip.com.au