Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

24V Latch Circuit for Momentary Switch

Status
Not open for further replies.

mtaylor

New Member
Hi,
I have a control unit which I want to use to switch a Hydraulic Solenoid. unfortunately the available switch in the control unit is momentary and I need the output to latch (so that I don't need to keep my finger on the button all the time) I had considered using a flip-flop to do this but I need to use a 24V supply. Are there any Flip flops which can take 24V? the highest I can find is 18V. I intend to use a relay to work the solenoid so the current rating is not important. I could use a volt reg to drop the voltage to a manageable level but I need this circuit to be as small (and simple) as possible. Does anyone have any ideas how to do this?

If the control lunit wasn't sealed, I'd change the switch but I don't want to break the seals.

Ta Much
Mike
 
Latching isn't a problem, a simple SCR can do that, it's unlatching which is more difficult (but you didn't mention a requirement for that?).

Overwise, a flip flop is probably the way to go, and a driver transistor to feed the relay, or even the solenoid directly.
 
Or use a latching relay circuit.
 
Sorry I forgot to put that bit in. I need it to un-latch too.

I want to be able to press the button once to switch the solenoid on then press it again to switch it off.

Does this mean that the flipflop is the best option? I like the idea of the transitor to feed the relay though that never occured to me!!!

Thanks for the advice. If you've any more I'm all ears.
 
If you search my posts, you will find that I designed a relay flip flop for someone a year or 2 ago. If you can't find it, let me know and I'll re-post.

A relay flip flop may be more convenient in your situation than an electronic one.
 
Nigel Goodwin said:
Good point! :D

However, the same consideration as an SCR applies, that it doesn't unlatch again - but due to lack of information, we don't know if that's OK or not?.

How about a normally closed push button in series with the coil?

Or a push to make in parallel with the SCR?

The latter is a pain as the switch needs to be rated the same as the SCR, but the relay isn't too bad - both switches only need to be rated for the coil current.
 
Hi Hero,
Yes I think that would work but I don't want to have to add more switches (originally I was going to just fit another (latching) switch to do the job but it would be much tidier if I could use the actual controller).

Thanks.
 
A bistable latching relay would be just what you want, as one impulse latches it, and then one impulse unlatches it. A typical one is the S89 series by TYCO(Potter and Brumfield).
If you want to use two switches one to latch and one to unlatch there are two approaches. As Nigel has mentioned a SCR. One normally closed switch between cathode and common of SCR. One normally open between gate and anode(with a resistor). Pressing the normally open will latch the SCR, Pressing the normally closed will unlatch the SCR.
The same technique can be used with a DPDT conventional relay, except a set of the NO contacts will latch the relay. If you need a circuit diagram I will be glad to provide one.
 
Last edited:
This Latching relay thing sounds like just the thing. I think I'll get one and give it a go. Many thanks.
 
mtaylor said:
Hi Len,
I can't find your post for the relay flip flop. Can you please post it again?
You did not say how you want to release the solenoid. If it is to be released by another button, then all you need is to latch the relay with one button and open the circuit with another.

This one will latch at the first button press and release at the second press.

Note that the operate and release times of the relays need to be at least 10 ms otherwise they may respond to contact bounce.
 

Attachments

  • Relay Flip Flop.gif
    Relay Flip Flop.gif
    6.7 KB · Views: 768
Last edited:
https://www.qprox.com/products/qt1xx.php

I use these chips a lot, they are touch sensor ic's that have an on/off switching mode, they are like $4. Very few components to build it, and you'll have a touch sensitive latch you can operate without a mechanical switch---maybe a piece of metal any size you want. Send the output to a mosfet on the low side of the solenoid. Maybe 5 bucks in parts excluding how fancy you get with the touch panel.
 
Thanks for all the ideas. I'm going to go with the Latching Relay because I've found somewhere I can buy on locally (it's called an Impulse Relay but it seems to be the same type of thing). Also it's the simplest option to install. If that doesn't work then I'll use the 2 relay circuit as it looks ideal, thanks Len. (it's a second press of the same button to release the solenoid).

The touch sensor ic is an interesting idea although I cannot use it for this as I want to use my existing momentary switch. I want to find something I can use this for though!

I'll let you all know how I get on!
 
Mike, if you want to use the 2 relay circuit with a SPST button rather than a SPDT one as drawn, I can show you how to do it, but it needs a transistor and 2 or 3 resistors.
 
panic mode said:
it is possible to do it if button has just one contact.
doesn't use any electronics but it needs three relays:
https://www.plctalk.net/qanda/showpost.php?p=80673&postcount=31
I agree, however a transistor and a few components may be cheaper and consume less PCB space than another relay. Another consideration is that the contact transit time of the third relay may be excessive if it has a diode across the coil to suppress the back EMF. So the diode may need a rresistor in series.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top