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I would like to make an alarm that goes off about 1 minute after it has been triggered. To do this, I want to delay the circuit using a 555 timer in monostable, but I can't do this as they turn it on for a certain amount of time and then off. Is there a way I could set up the 555 so that this is reversed? (I can't use a NOT gate as then it would be on while not being triggered) Or maybe another way this can be done?

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    \$\begingroup\$ Its's not clear what type of signal you are trying to achieve. You want it to be low by default and then high, or the inverse? \$\endgroup\$
    – clabacchio
    Nov 19, 2014 at 10:20
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    \$\begingroup\$ Can you draw a timing diagram of what you ae trying to do? \$\endgroup\$
    – tcrosley
    Nov 19, 2014 at 11:14
  • \$\begingroup\$ 1. What is your operating voltage? 2. will your trigger be momentary or continuous? 3. Will your trigger be high-going or low-going? 4. What load will the delayed output see? 5. Will you need a manual reset for the output or will power-down shut it off? \$\endgroup\$
    – EM Fields
    Nov 19, 2014 at 14:18

2 Answers 2

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I think if you use a second 555, also connected as a monostable, but replace the timing capacitor with a short (and you don't need the timing resistor)-- triggered by the first one, that should work.

You could also use a D flip-flop such as a 4013 with the D tied high, but you'd need an inverter on the clock and a power-on reset circuit.

Or, something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

C1/R3/D1 provides a semi-reliable power-on reset (100% reliable if the power turns on cleanly every time after a relatively long 'off' time where +V falls to near 0V).

C2 and R4 provide an edge trigger from the 555 output falling edge.

R1 and R2 protect the CMOS inputs

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I solved it using random circuit digrams off Google Images, thanks for the answers. This is the circuit diagram I solved it with.

schematic

simulate this circuit – Schematic created using CircuitLab

The output goes to rest of the circuit, and discharge is not connected. This produces a low output for around a minute and a high output for an unlimited time. Thanks again.

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