Requesting help on how to terminate a ground connection AFTER a designated time period.

Thread Starter

BigCircuit3302

Joined Sep 2, 2022
19
I am for suggestions on how to accomplish the following task........

In the provided diagram, I show a momentary botton switch that connects the base of 2 pnp transistors to ground when it is depressed.
What I need help on is this.........
If the button is depressed for MORE than a desired proof of time (10 sec. for example), I would like the ground connection to base of the pnp's terminated EVEN IF THE BUTTON REMAINS DEPRESSED.
Whatever method is used to accomplish this task would need to automatically "reset" after being triggered. The reset time period would be somewhere between 20 and 30 seconds.

I thank you for your suggestions on this matter.
 

Thread Starter

BigCircuit3302

Joined Sep 2, 2022
19
I am for suggestions on how to accomplish the following task........

In the provided diagram, I show a momentary botton switch that connects the base of 2 pnp transistors to ground when it is depressed.
What I need help on is this.........
If the button is depressed for MORE than a desired proof of time (10 sec. for example), I would like the ground connection to base of the pnp's terminated EVEN IF THE BUTTON REMAINS DEPRESSED.
Whatever method is used to accomplish this task would need to automatically "reset" after being triggered. The reset time period would be somewhere between 20 and 30 seconds.

I thank you for your suggestions on this matter.
attachment.........
What diagram?
l tried to add atyachm
 

Attachments

ronsimpson

Joined Oct 7, 2019
3,145
In the diagram there is "IC chip" pins 2 & 12. Please attach the schematic. I think this can be done with less parts, but I can't see the whole picture.
 

Thread Starter

BigCircuit3302

Joined Sep 2, 2022
19
In the diagram there is "IC chip" pins 2 & 12. Please attach the schematic. I think this can be done with less parts, but I can't see the whole picture.
Please don't be concerned with any other part of this diagram except for what I described with the button switch........
I'm just trying to add a safety feature to an existing circuit.

Thanks
 

Tonyr1084

Joined Sep 24, 2015
7,959
a momentary botton switch that connects the base of 2 pnp transistors to ground when it is depressed. • • •
If the button is depressed for MORE than a desired proof of time (10 sec. for example), I would like the ground connection to base of the pnp's terminated EVEN IF THE BUTTON REMAINS DEPRESSED.
Whatever method is used to accomplish this task would need to automatically "reset" after being triggered. The reset time period would be somewhere between 20 and 30 seconds.
By "Push button" and "2 PNP Transistors" I assume you're talking about this part of the circuit:
Screenshot 2024-03-30 at 9.33.21 AM.png
If you're building this circuit, change from PNP to NPN. Use a pull-down resistor to hold them off. The momentary button can be pressed and provide power through a large capacitor and resistor to achieve the time frame you want. When you push the button the transistors will turn on until the cap charges up to approximately 66% (if I have my cap facts right). When the cap charges the transistors will switch off. During the charging phase the transistors will be on in varying levels. Since the transistors are powering relays, the relay will add some dimension to the timing of the circuit. A relay rated for a certain voltage will need a minimum percentage of current to cause the relay to latch. It will hold until some lower percentage of current reaches a point where the relay drops out.

Use of transistors versus using MOSFET's, Fets would be better as they have a quicker response and smaller period of going through the switching phase from off to on. An N-type MOSFET should do the job better than a PNP transistor.
 

ronsimpson

Joined Oct 7, 2019
3,145
Please don't be concerned with any other part of this diagram
What happens for the time it takes for the relay to switch positions? There will be a time when no resistor is connected. I know it is just a fraction of a second but in some circuits that is a long time.
If you're building this circuit, change from PNP to NPN.
Doing that requires the addition of a diode across the relay.
attachment.........
Why do you need relays? I think it can be done without.
 

crutschow

Joined Mar 14, 2008
34,692
Below is the LTspice sim of a 555 circuit to control the PNP transistor, if you want to minimize changes to the circuit:
The only change otherwise is the push-buttons are connected to V+ instead of ground.

The relay turns off when the PB (V(s)) is released before 10s (green traces) (V(rel) and V(out) low is on), but turns off after 10s (yellow traces) if the PB is held longer than that.

1711817965273.png
 

MisterBill2

Joined Jan 23, 2018
19,024
OK, for the circuit shown, a series capacitor can be inserted in series with the switch. The charging current will pull down the transistor bases untilit charges enough, at which point the transistors will switch back off. Yo can indeed replace the transistors with FET devices and need much less capacitance. That is about as simple as I can do it.
My scheme is vastly simpler that the one in post #10 and it draws no standby current at all.
 

Thread Starter

BigCircuit3302

Joined Sep 2, 2022
19
I'd like to thank everyone for their input on this topic. Suggestions to change PNP to NPN or MOSFET'S and change this and change that which is all good, however that is not what I'm asking for. I would like an answer to my question regarding the negative ground connection delivered to the base of the PNP transistors via the button switch. I have described in detail what it is I am trying to do and would like input on accomplishing what I have asked for.
Suggestions on anything other than this is not necessary.
Remember, the heading for my post is:
Requesting help on how to terminate a ground connection AFTER a designated time period.

Thank you very much for your efforts.
 

crutschow

Joined Mar 14, 2008
34,692
So what degree of changes can you tolerate?
It reminds me of the old request from management: "Fix it but don't change it".

Below is the sim of MB2's suggested circuit, which is about the minimum to meet your delay requirements:
You may have to change the value of C1 to get the delay you want.

R1, R2, Q1, and U1 are from your circuit.
C1 and D1 are added.

1711824308484.png
 

MisterBill2

Joined Jan 23, 2018
19,024
I'd like to thank everyone for their input on this topic. Suggestions to change PNP to NPN or MOSFET'S and change this and change that which is all good, however that is not what I'm asking for. I would like an answer to my question regarding the negative ground connection delivered to the base of the PNP transistors via the button switch. I have described in detail what it is I am trying to do and would like input on accomplishing what I have asked for.
Suggestions on anything other than this is not necessary.
Remember, the heading for my post is:
Requesting help on how to terminate a ground connection AFTER a designated time period.

Thank you very much for your efforts.
That was the main point of my suggestion, which is to add a series capacitor to the switched ground line. That change, all by itself, will create the same function as releasing the button after some time. Try a 470 MFD capacitor with at least a 12 volt rating and see how much time that gives you. Worst case might be that you would need to change the 1K resistors between base and emitter to 10K resistors. But maybe not. Adding the capacitor in series will cause the effective release after some period of time, I did not do the math or simulation to see how much. But certainly the effect will be what you requested with only one part added.
 
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