0
\$\begingroup\$

I'm really struggling to find a simple circuit that'll give me a pulse (~1s) when switching the circuit on. The latching switch can't be changed and already controls other circuits, and the new module/controller requires a short pulse to turn on, but will reset if the on control input stays on for more than 10s.

Edit to add more info - Voltage when latching switch is on is 12V and 0V when off, there can be a constant 12V feed if needed to power circuitry but this'd just be pulled from the back of the switch. The new module requires a ~1s high (12v) pulse to tell it to turn on. Much less than that and it misses the on command sometimes.

I'll see if I can get a schematic to upload later as can't at the momentEdited again to add bad phone picture of schematic

Edit to add more info: pulse current - no more than 50mA. The reset time could be as low as ~200ms in theory but if it's a limitation ~1s would be acceptable

I'm not 100% sure but I believe that as long as the pulse is at least 9v it should work

\$\endgroup\$
  • 1
    \$\begingroup\$ Can you provide a sketch or schematic of your setup? In particular, we need details. What is the voltage level when the latch output is ON? What is the voltage level when the latch output is OFF? Are your control signals active HI or active LO? \$\endgroup\$ – Dwayne Reid Jan 14 '19 at 16:28
  • \$\begingroup\$ are you sure that your controller needs 1s of pulse? That sounds incredibly long. There's Linux systems that boot in that time. \$\endgroup\$ – Marcus Müller Jan 14 '19 at 16:31
  • \$\begingroup\$ How much current do you need from the one-shot timer? \$\endgroup\$ – Dwayne Reid Jan 14 '19 at 16:55
  • \$\begingroup\$ How quickly does the one-shot timer need to reset? That is - what is the minimum OFF time from the switch? \$\endgroup\$ – Dwayne Reid Jan 14 '19 at 16:57
  • \$\begingroup\$ Does the pulse output have to be 12 V, or can it be around 10.5 V? This can be done with one CMOS 555, or a couple of transistors, or one XOR gate, or a Schmitt trigger NAND gate like a CD4093. \$\endgroup\$ – AnalogKid Jan 14 '19 at 17:16
1
\$\begingroup\$

A simple CMOS 555 timer chip will do the trick for you. There are many other techniques but this one is simple. Something like a TLC555 is good up to 18 Vdc and can supply a maximum of 200 mA without damage.

Circuit is dirt simple. Do note that the capacitor discharge depends on there being a load after the switch to provide a discharge path to ground.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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