2
\$\begingroup\$

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?

\$\endgroup\$
2
  • \$\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
    Commented Dec 18, 2015 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
    Commented Dec 18, 2015 at 23:30

1 Answer 1

Reset to default
6
\$\begingroup\$

schematic

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.
\$\endgroup\$
3
  • \$\begingroup\$ Good answer. Not all thgose inverters are Schmitt AFAIR. Schmitt includes 74x14, CD40106, ... . This sort of circuit works very well. \$\endgroup\$
    – Russell McMahon ♦
    Commented Dec 19, 2015 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
    Commented Dec 19, 2015 at 1:01
  • \$\begingroup\$ Wow, thanks a lot guys. Seems simple enough to implement. Appreciate it. \$\endgroup\$
    – Jim
    Commented Dec 21, 2015 at 15:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.