Voice operated recording (VOR) is 411 a feature found on most handheld memo-recorders. The function of the VOR control is to start the recorder’s tape on detection of a speech signal. This relieves the user from pressing a ‘record’ button. Typical memo-recorder users include the too energetic manager and the detective on TV, unraveling a murder plot by talking to himself behind the wheel of his fast .sportscar. The circuit shown here adds a VOR function to any standard cassette recorder, provided this has a ‘remote’ control input. Alternatively, it could be used as a VOX (voice operated switch) to control a transmitter or a sensitive sound detector in an alarm system. Depending on the position of switch S1, the output signal from the electret microphone (approx. InY) is amplified x520 or x2,200 by opamp IC1, which also functions as an active filter with a (speech) bandwidth from 160 Hz to aboUt.0 kHz. Components DI, C5, and R8 turn the Amplified audio signal into a direct voltage, which is applied to the inverting input of comparator IC2a. If the sound picked up by the microphone is sufficiently loud, the voltage at the inverting input falls below that at the non-inverting input, and the comparator output changes state. The resulting logic high at pin 7 of IC2a causes relay Rei to be energized so that its contact closes. If the contact is connected to the remote control input on the recorder, the tape will be started, and recording begins.
This condition is indicated by the lighting of D2. The supply voltage to the circuit and the microphone is switched on and off with S3. Switch S2 allows the VOR function only to be switched off, while S1 serves to select between ‘low’ and Very high’ sensitivity. Since the gain-bandwidth product of the LF357 is about 20 MHz, the opamp will have a bandwidth of about 9 kHz if its amplification Is x2,200. With S1 closed, the gain is reduced to x520 times, which results in an effective bandwidth of 38.5 kHz. Since this is not practical or desirable for s’)each signal, capacitor C4 is included in the feedback loop to give a bandwidth of 9 kHz at the lower gain also. Resistor R12 prevents the opamp oscillating by maintaining a minimum amplification of x5.
When the relay is energized, the circuit draws about 18 mA from the 9 V battery; if not, the current is roughly halved. The minimum battery voltage for reliable operation is about 6.8 V.
The VOR is an extremely sensitive circuit, and to ensure that it cannot be upset by strong RF signals, it is best built on the printed circuit board shown here. The dimensions of the board allow it to be fitted into a ready-made plastic enclosure with an integral battery compartment. A suggestion for a nice looking front panel is also given. Notice that all inputs and outputs of the circuit are located at one side of the PCB – this is done to ensure that com-mon-mode signals induced in the cables are not ‘forced’ to cross the PCB, where they could affect the operation of the electronics. All connections to ground are soldered to a wide copper pad that forms a low impedance. An exception has been made in the case of capacitor C5: to prevent the discharge currents interfering with the microphone signal, they should not be allowed to travel through the ground pad.
Parts list
Resistors:
RI = 330 (ohm)
R2 = 1.5 k(ohm)
R3, R4 = 47 k(ohm)
R6 = 2.2 M(ohm)
R7 = 680 k(ohm)
R8= 5.6 M(ohm)
R9= 470 k(ohm)
R10= 390 (ohm)
R11 = 3.3 M(ohm)
R12 = 5.6 k(ohm)
R13 = 100 (ohm)
Capacitors:
C1, C7 = 47 uF, 16V
C2= 470 nF
C3= 1 uF,16 V
C4= 33 pF
C5 = 2.2 uF, 16V
C6= 100 nF
C8 = 10 uF, 16 V
C9 = 14 nF
Semiconductors:
D1, D3 = 1N4148
D2 = LED, red, 3 mm
Integrated circuits:
IC1 = LF357
IC2 = TL072
Miscellaneous:
Si, S3 = AS1D- 5M single-pole change-over slide switch angled wires. PCB mount (Fujisoko)
S2 = AS2D-5M double-pole change-over slide switch, angled wires, PCB mount (Fujisoko)
X1 = MCE-101 electret micro-phone (Mona-cor/Monarch)
Re1 = relay, 5V/3800; (Siemens V23100-4111 V4005-A001)
1 Plastic case; dim. 61*97*26 mm (PacTec Type HM-kit)
1 9-V battery clip
1 Printed circuit board 934039
