General-Purpose Astable Multivibrator Circuit
This circuit functions as a versatile astable multivibrator, cyclically energizing two substantial loads through a relay, typically 12-V incandescent lamps in this instance. Unlike traditional ‘analogue’ flip-flops, there is no requirement for power transistors with heatsinks. Consequently, this alternating blinker can be constructed at a reduced cost, with greater ease, and in a more compact form.
Charging and Discharging Sequence for Relay Activation
In its idle state, capacitor C1 undergoes charging through R1 and simultaneous discharging through R2 and P1. Proper adjustment of P1 is crucial to ensure sufficient current for triggering transistor T1, typically occurring when the capacitor voltage reaches around 1.2 V if a BC517 transistor is used. Consequently, the relay activates.
Controlled Relay Deactivation and Restart
Upon relay activation, R3 and P2 are connected in parallel with R2 and P1. Precise adjustment of P2 is necessary to restrict the remaining current, preventing the provision of base current for T1. This leads to a voltage drop across C1, eventually cutting off the transistor. Consequently, the relay deactivates, initiating a new cycle of operation.
Power Supply and Circuit Protection
The circuit can be powered by an unregulated 12-V mains adapter. Current consumption primarily depends on the loads, as the alternating blinker circuit only draws the relay’s rated current. Each load is directly connected to the supply voltage, while the blinker circuit receives a stabilized supply via the fixed voltage regulator IC1. Diode D1 safeguards the circuit against incorrectly polarized input voltages.
Calibration and Component Selection
During setup, adjust P1 to its minimum and P2 to maximum resistance. Gradually increase P1 until the relay engages, repeating the process with P2 until the relay disengages. This procedure allows control over both the blinking rate and the desired on/off ratio. The author used a BC337-40 for T1; however, a BC517 (Darlington) can be substituted if BC337-40 is hard to find. The circuit’s performance is also influenced by the type of relay employed. In cases where the relay behaves erratically, especially if you rely on a specific relay type, adding capacitor C2 (as indicated in the dashed outline) might help slightly delay the effect of switching in P2 and R3.