Parts List:
R1 = 1 Meg, Preset Pot
R2 = 10K
R3,R4 = 1K
C1 = 10uF, 16V
C2 = 0.01uF
T1 = BC547 (Gen Purp NPN)
T2 = 2N2222 (Hi Current NPN)
D1 = 1N4001 (Gen Purp Si)
IC1 = 555 (Lo-Power version)
RLA1 = Relay, 9V (amps of your choice)
Circuit Operation:
IC1 is wired as a monostable multivibrator. Input pulses are applied to the base of transistor T1.
This causes T1 to conduct, taking pin 2 of IC1 'low'. IC1 then enters a timing cycle, the duration of which is et by R1/C1, causing pin 3 of IC1 to go 'high'. This causes T2 to conduct.
At the end of the timing cycle T2 switches off, and the circuit waits for the next pulse to T1. T1 replacements: ECG123AP or NTE123AP (Rotate device 180° to conform with original lead configuration). T2 replacements: ECG123A or NTE123A.
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