The transmitter is intended primarily for use with the receiver in the preceding articles. It works form two small 1.5 V dry batteries or one 3 V lithium battery. To make the unit compact and yet have reasonable frequency stability, the design uses a phase shift oscillator, T1, which provides good frequency stability. To ensure that the transistors have sufficient amplification at the low supply voltage, it is a darlington. Each of the branches of phase shift network.
R1-C1-R2-C2-R3-C3 has roughly the same time constant. To ensure that the branches do not affect each other unduly,
R2 = 3.8 R1, and R3 = 3.8 R2.
Because of this mutual influence, the gain of the amplifier is somewhat higher than the theoretical value of 18 dB.
The value of R5 is a compromise, it needs to be low so as not to affect the phase shift network the output impendence of T1 also affects the oscillator’s frequency.
At the same time, it must not be too low, because that would increase the current and reduce the gain.
To lessen the influence of the base-emitter resistance of buffer T2. This transmitter is driven by T1 via an independent resistor, R6.
The buffer is necessary to provide vide sufficient current for the LEDs. Owing to the low supply voltage. these diodes cannot be connected in series. and they are, therefore. driven independently via their own series resistor. R7 and R8.
The transmitter is switched on and off by connecting or disconnecting the supply voltage with S. A drawback of this is that current continues to flow as long as Si is pressed. Briefly pressing of S1 is. however. sufficient to switch the receiver on or off.
The current drawn by the circuit depends on the supply voltage and on how long S1 is pressed. When Ub = 2 V, the frequency is 29.3 kHz, the peak current through each of the LEDs is 25 mA, and the total current drain is 27 mA. When Ub = 3.2 V. the frequency is 30.4 kHz. the peak current through each of the LEDs is 64 mA, and the total current drain is 63 mA. At this supply voltage. the range between the prototype transmitter and receiver was 13 m (43 ft).
The current drain may be reduced by connecting in series with S1 is parallel RC network (R = 10 KΩ:C = 1000 uF, 6.3 V). When Si is then pressed, only a brief current pulse ensues: even if S1 is held down, the current does not rise above about 300 uA.
