# How to keep this latching circuit locking?

Joined Nov 21, 2018
709
Hi,
The circuit attached is a self-locking circuit, the problem is when the trigger pulse trigger the circuit make the relay ON, but the trigger pin itself goes to 0 after the pulse, so the circuit goes close again, how to keep the circuit ON until next pulse?
Thanks

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#### LowQCab

Joined Nov 6, 2012
1,635
There are no values shown for the components.
At first glance, the Circuit should work as expected.

Have You actually built the Circuit and tested it ?

What does the Input Circuitry look like ?

R-1 could be too low in Resistance,
R-3 could be too high in Resistance.

Rather than trying to figure out a Circuit that you found,
and that You think will work,
please let us know what the purpose of the Circuit is.

Why are You using a Relay ?
.
.
.

#### DickCappels

Joined Aug 21, 2008
8,180
I'm lazy sometimes (ok, a lot of the time). How about just putting a diode in series with R1 so that R1 does not sink current, only source it to the base?

#### Ian0

Joined Aug 7, 2020
4,169
If
$$V_{supply}.\frac{R1}{R1+R3} > 0.6$$
then it will stay on.
Otherwise, grounding the input will switch it off.
If you can't switch it off by grounding the input, it will stay on until the power supply is switched off.

Joined Nov 21, 2018
709
There are no values shown for the components.
At first glance, the Circuit should work as expected.

Have You actually built the Circuit and tested it ?

What does the Input Circuitry look like ?

R-1 could be too low in Resistance,
R-3 could be too high in Resistance.

Rather than trying to figure out a Circuit that you found,
and that You think will work,
please let us know what the purpose of the Circuit is.

Why are You using a Relay ?
.
.
.
Thank you.
I put the link of the circuit in the main post.
I built the circuit and its works as my post.
The purpose of the Circuit is to prevent the remote control car go craze. the relay can cut power.

Joined Nov 21, 2018
709
I'm lazy sometimes (ok, a lot of the time). How about just putting a diode in series with R1 so that R1 does not sink current, only source it to the base?
Thank you and I'll test it.

Joined Nov 21, 2018
709
If
$$V_{supply}.\frac{R1}{R1+R3} > 0.6$$
then it will stay on.
Otherwise, grounding the input will switch it off.
If you can't switch it off by grounding the input, it will stay on until the power supply is switched off.
Thank you.
I need it power off at next pulse, like many one button on/off circuit. seems this circuit can't get there.
like this one:
https://www.instructables.com/Three-Push-ON-Push-OFF-Latching-Circuits/
is it possible changed into trigger-lock circuit?
Thanks

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#### Ian0

Joined Aug 7, 2020
4,169

#### crutschow

Joined Mar 14, 2008
28,580
Below is the LTspice simulation of a triggered bistable circuit using an IC flip-flop.

The R1-C1 feedback delay provides switch-bounce protection.
It's not needed if the signal is a clean pulse (just remove R1, R3, and C1, and connect the /Q output directly to the D input).

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#### MisterBill2

Joined Jan 23, 2018
10,602
In the circuit of post #1, it seems that the value of resistor R3 is to high to provide adequate base drive to Q1 to keep it switched on. And it appears that the input to R1 is being pulled low when the latch pulse is removed. It may also be that Q2 does not have enough gain to fully power the relay, and raise the emitter voltage high enough.

Joined Nov 21, 2018
709
Below is the LTspice simulation of a triggered bistable circuit using an IC flip-flop.

The R1-C1 feedback delay provides switch-bounce protection.
It's not needed if the signal is a clean pulse (just remove R1, R3, and C1, and connect the /Q output directly to the D input).

View attachment 248837
Thanks.
I an still looking for a trigger lock circuit.
I am trying to build this one, can I use a CD4066?

#### ericgibbs

Joined Jan 29, 2010
14,729
Ref post #1

What is the value of R1 resistor and what is the Trigger input connected too.
Is the input trigger pulled LOW by the driving source.? after the +V pulse
E

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#### MisterBill2

Joined Jan 23, 2018
10,602
The circuit in post #1 does have positive feedback and so with some combination of resistance values it should latch to the "ON" state with a positive input from a source that has a high impedance when it is in the low state. So if the TS has a simulator it would just be a matter of experimenting to find the correct values for the resistors.
R4 must have a high value because iit only needs to compensate for the idle current when the base of Q1 is low. R2 must have a low enough resistance to keep Q2 saturated , or close to saturated, when Q1 is conducting because of the base current supplied by the input pulse and the current through R3. R3 must be a low enough resistance to allow enough base current for Q1 to stay conducting when the pulse is removed. R1 must have a low enough resistance to reduce the base current on Q1 when the input is held low.
With different resistance values the circuit will operate like a comparator, but not as fast or as stable.It would switch the relay load quite reliably as the input voltage would rise and fall. That may not be the form of latching that the TS is seeking, we have no comment about that.

#### Ramussons

Joined May 3, 2013
1,155

#### MisterBill2

Joined Jan 23, 2018
10,602
After reading the first post again, I see that the TS wants alternate actions, pulse on, next pulse off. The posted circuit will not provide that function with any set of resistor values. It only can provide switch ON with a voltage pulse and switch off with a pull down to common pulse.
The flipflop will provide that function very well, the initial circuit can not do it at all.

#### crutschow

Joined Mar 14, 2008
28,580
I an still looking for a trigger lock circuit.
Why do you think the FF circuit I posted won't do what you want?

Joined Nov 21, 2018
709
Why do you think the FF circuit I posted won't do what you want?
I am trying to build this one, can I use a CD4066?
I don't have the parts handy now.

#### crutschow

Joined Mar 14, 2008
28,580
I am trying to build this one, can I use a CD4066?
Which one?
A CD4066 is a CMOS switch, so it's not clear how you would use that to do what you want.

#### DickCappels

Joined Aug 21, 2008
8,180
Is there any problem with using crutschow's circuit in post #9? It looks like it fits the bill to be.