Compact Transmitter Design for Use with the Receiver
Basic Infra-Red Transmitter: This transmitter is primarily intended for use with the receiver discussed in the preceding articles. It is powered by two small 1.5 V dry batteries or a single 3 V lithium battery. The design aims to maintain compactness while ensuring reasonable frequency stability. To achieve this, a phase shift oscillator, T1, is employed, providing commendable frequency stability. The Darlington configuration is utilized to ensure sufficient transistor amplification at the low supply voltage. Each branch of the phase shift network, R1-C1-R2-C2-R3-C3, is designed with roughly the same time constant. To minimize undue mutual influence among the branches, the resistor values are set at R2 = 3.8 R1 and R3 = 3.8 R2, resulting in a slightly higher amplifier gain than the theoretical 18 dB.
Optimizing Resistor Values for Performance
The value of R5 in the design is a compromise, balancing the need for a low value to avoid affecting the phase shift network. The output impedance of T1 plays a crucial role in the oscillator’s frequency, and it must strike a balance—not too low to avoid increased current and reduced gain. To mitigate the influence of the base-emitter resistance of buffer T2, the transmitter is driven by T1 through an independent resistor, R6.
Buffer for LED Current and Transmitter Switching
A buffer is essential to ensure an adequate current supply for the LEDs due to the low supply voltage. These diodes cannot be connected in series because of the low voltage, so they are independently driven through their respective series resistors, R7 and R8. The transmitter’s power is toggled by connecting or disconnecting the supply voltage using switch S. A drawback of this method is that current continues to flow as long as S1 is pressed. However, a brief press of S1 is sufficient to turn the receiver on or off.
Current and Range Characteristics of the Transmitter
The circuit’s current draw depends on the supply voltage and the duration of S1 being pressed. For Ub = 2 V, the frequency is 29.3 kHz, the peak current through each LED is 25 mA, and the total current drain is 27 mA. With Ub = 3.2 V, the frequency becomes 30.4 kHz. The peak current through each LED is 64 mA, and the total current drain is 63 mA. At this supply voltage, the range between the prototype transmitter and receiver measured 13 m (43 ft).
Reducing Current Drain with Parallel RC Network
To mitigate current drain, a parallel RC network (R = 10 KΩ:C = 1000 uF, 6.3 V) is connected in series with S1. When S1 is pressed, only a brief current pulse occurs. Even if S1 is held down, the current remains below about 300 uA.