Figure 1 shows a simple audio oscillator driven by the SCR When the sensor is closed. The circuit can be easily mounted using a terminal strip as a chassis. Observe the position of the polarized components as the transistors and power supply lines.
Q1 - BC548 or equivalent general-purpose negative-positive-negative (NPN) silicon transistor
Q2 - BC558 or equivalent general-purpose positive-negative-positive (PNF) silicon transistor
P1 - 100 k - trimmer potentiometer
R1 - 10 k x 1/8-watt resistor (brown, black, orange)
R2 - 1 k x 1/8-watt resistor (brown, black, red)
C1 - 100 µF x 12-volt electrolytic capacitor
C2 - 0.047 µF ceramic or polyester capacitor
SPKR 4- or 8-ohm small speakers (2.5 to 10 cm)
Terminal strip, box, wires, solder, etc.
