switch 'latch' circuit

[greggreg]

I'd like to latch closed a momentary switch (norm open). In a 12 v car environment operating a elect. actuator. I'd like it to stay closed until it is pressed again (to stop movement) or until the actuator stops (actuator will stop at full extension). Ideally it would also 'lock out' the other two poles of the switch(switch is a DPDTmom) as hitting the switch into down while it was latched in up operation would short out power. (Switch acts as a polarity reverser for up/down operation.) Anyone know of an existing circuit or have a design they could throw out? Note:I want the 'down' switch to not 'latch' as there is a serious pinch hazard as the hatch comes down - no such issue going up!
THX!

[Thomas W]

Do you want to use an existing DPDT switch, or do you want to build your own electronic switch?

[greggreg]

Using existing dpdt switch mom (carling exterior boat switch). It matches with another functional switch - and it's a sweet looking lighted switch!

[greggreg]

I did find one I think I can make work ( http://www.the12volt.com/relays/relaydiagram24.html) but it uses 5(!) spdt relays and I'll probably need one more to 'lock out' reversing the actuator when it is latched. Jeeze, 6 relays! Looking for something a little more elegant than a wall of relays!

[Thomas W]

Is this a correct block diagram of your system?:

switch_latch_circuit.png (10.14 KiB) Viewed 15811 times

(Without discussing what should actually go into the 'New Circuit' block yet )
There are probably many ways to do this, but would this be okay?

[greggreg]

switch.png (168.07 KiB) Viewed 15776 times

Here's the circuit (not much ) without a 'latch' circuit. As can be seen it simply reverses polarity to operate actuator up/down. I'd like to insert a circuît in line with the 'up' connection which latches the power. I'm open to other configurations as long as I still use the dpdt switch I have. In other words, I only want the latching circuit to affect the 'up' operation - down operation should be momentary (only operate with switch manually pushed) for safety reasons. You can also easily see why I want to disable a 'down' button press when the 'up' is latched so I don't short things out.

[Thomas W]

So what you've got already is this circuit:

switch_latch_circuit_2.png (6.6 KiB) Viewed 15754 times

(I forgot to make it a 'momentary' switch).
I not sure if the new circuit below is 'elegant' but it should work :

switch_latch_circuit_3.png (50.55 KiB) Viewed 15754 times

Your DPDT momentary switch is reused (S1). The polarity reversal of the actuator is inspired by an H-bridge: http://en.wikipedia.org/wiki/H_bridge
The 555 is configured as an electronic toggle switch. Each time the physical momentary switch is pushed 'up' the output of the 555 changes state (ON -> OFF -> ON etc.) T1 and T2 are relay drivers. T3 resets the 555 into OFF state, when S1 is pushed 'down', and RL2 is turned ON, with a little delay due to C3.
D1 and D2 are protecting T1 and T2 from overvoltage when RL1 and RL2 turn off.
F1 is a fuse, just in case the concept won't work, and RL1 and RL2 are turned ON at the same time and short circuits +12 V to 0 V.
R3 and C1 are providing a better supply for the 555, but might not be needed, depending of the quality of the +12 V supply.
R1 (x2 - oops ) and R7 are limiting the current into T1, T2 and T3.
The 555 toggle switch concept is described here: http://www.bowdenshobbycircuits.info/page9.htm "Relay Toggle Circuit Using a 555 Timer".

[greggreg]

Looks good. I'll give it a go when I get a chance (on vacation, class for couple weeks). I probably need to smooth the power source a bit, 12v from my truck is the source. Question, any recommendations on transistors to use? This circuit will live inside a cargo trailer - very hot/cold. Also, I'll probably use some conformal coating to protect board/wiring from moisture, etc. A good source for the small amount I'll need?
Again, Thomas, thanks. I'll let you know how it works!

[Thomas W]

You're welcome, Greg - looking forward to hearing about your results
The size of the transistors T1 and T2 depends on the choice of relay for RL1 and RL2, i.e. how much current do the relays need in order to operate? [mA]
T3 could be a P2N2222 or BC547/BC548.

D2 was flipped by accident in switch_latch_circuit_3.png, so I have updated the schematic (and R1 has been renamed to R8):

switch_latch_circuit_4.png (50.37 KiB) Viewed 15597 times