I have a situation where I'd like to delay the closing of an off the shelf relay by ~5 seconds. Assuming it is a 5V coil, and switching 12 V, I'd like the following:

t 0:00: 5v applied to input, relay switch open. t 0:05: coil closed, relay switch closes.

I think some simple RC network may do the trick, but I'm struggling to imagine the right configuration. Any ideas?

  • \$\begingroup\$ Yes, a RC does the trick, old stuff relay control boxes were doing this way, but I guess not for 5 seconds, it's quite long delay. \$\endgroup\$ – Marko Buršič Dec 18 '15 at 23:26
  • \$\begingroup\$ Off the shelf a "time delay relay" of the "on delay" flavor would do it. Or you can build something based on a timer, which is essentially an RC with some help. \$\endgroup\$ – Ecnerwal Dec 18 '15 at 23:30


simulate this circuit – Schematic created using CircuitLab

A CMOS schmitt trigger 40106 or 4093 (check the numbers) would work for this.

  • Pick a reasonable value for C1 - 10uF, say.
  • Calculate R so that R * C = 5 s.
  • When the input goes high C1 will charge up. At about 2/3 of supply voltage NOT1 output will switch low and NOT2 will switch high. Q1 will turn on and the relay will pull in.
  • D1 allows instant reset by discharging C1 when input goes low. It may need addition of a resistor to limit the current to whatever's driving the circuit.
  • D2 protects Q1 when RLY1 is switched off.
  • \$\begingroup\$ Good answer. Not all thgose inverters are Schmitt AFAIR. Schmitt includes 74x14, CD40106, ... . This sort of circuit works very well. \$\endgroup\$ – Russell McMahon Dec 19 '15 at 0:00
  • \$\begingroup\$ +1. I agree with Russell if you need a one-off and can trim R1 or C1 to compensate for the variation of NOT1's threshold voltage. Otherwise, I'd suggest you use a voltage comparator to turn Q1 ON and OFF where the trim will not be nearly as severe. \$\endgroup\$ – EM Fields Dec 19 '15 at 1:01
  • \$\begingroup\$ Wow, thanks a lot guys. Seems simple enough to implement. Appreciate it. \$\endgroup\$ – Jim Dec 21 '15 at 15:43

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