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timer 555 circuit once or continues loop?

mechie

New Member
The 555 Timer

All three of these 555 timers are connected as monostable multivibrators; so a single cycle will be seen rather than continuous. The switch [S1] simply kills the power to the monostab IC3 and the 74107 IC5.
This means that ICs 1&2 will time out soon after power is applied to the circuit and then sit dormant until power is isolated and restored :?: regardless of the position of S1.

The only way I can see this circuit working is if S1 kills the power to everything (an on-off switch) :?

Although this way of testing a 555 seems OK it doesn't check the discharge transistor. I wonder if a simple astable circuit would be more thorough - it would just mean watching an LED to observe flashing for OK or steady for faulty.
 

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