~Week 7~ Construct Transmitter And Receiver

"Transmitter"

The circuit diagram for a single transmitter units is shown below, current is the supply to the circuit from a 3V "coin" cell B1, via on/off switch S2 and diode D1. The diode provide reverse polarity protection.

It is best to use the specific schottky device which introduce a smaller forward voltage drop, and therefore loss, then a conventional silicon diode(0.2V rather than 0.7V approximately). Capacitors C2 provide a small reverse of energy and prevent the supply voltage from fluctuating. This stabilises the operation. 

A low power 7555 timer, IC1, is setup in a standard astable (pulse generator) configuration while switched on, this produces a continues train of on-off pulses at its output, pin 3. 

This Choice of resistor R1, R2 and Capacitors C1 provides one pulse per second for one of transmitter (Unit A) and one pulse every 1.2 second for the other one (unit B). In fact, the timing are slightly longer but it help to consider them as above. 

Beside that, The on time are much longer than the off one in each case. The purpose of these will be explained presently. 

(Transmitter Schematic Diagram)

"Receiver"

The complete circuit for the receiver section is shown below. This draw approximately 3mA, so a most substantial battery pack is needed than for transmitter. While the buzzer WD1 is sounding, the current rises to sound 6mA. 

The battery pack use in the prototype consist of four AAA size alkaline cell. If AA size cell could be accommodated inside the case, these would provide a doubling of operation time. 

(Receiver Schematic Diagram)