# circuit that acts as momentarily contact

#### Redge1

Joined Apr 25, 2020
17
I need a circuit that gives momentarily contact when I close a switch and will give another momentarily contact when I open the same switch again.So when I switch a switch on there must be a circuit that acts as a push button switch and when I switch that switch off again it must act as another separate push button.I have a device that switches on with a push button and switches off with another push button. I want to switch the device on and of with a thermostat by using a circuit that connects the thermostat to those two push buttons, so that i can still use them to switch the device if I want to.

#### Papabravo

Joined Feb 24, 2006
21,304
Double edge triggered one-shot

#### Redge1

Joined Apr 25, 2020
17

#### Papabravo

Joined Feb 24, 2006
21,304
Hi Papabravo, what does that mean?
You asked for a circuit and I told you the name of a circuit that I thought you were describing. Google is your friend and "it" will have no trouble providing a wealth of information including dedicated IC's that will perform the function. In the meantime:
1. A one-shot is the colloquial name for a circuit with the official name: Monostable Multivibrator. This somewhat archaic name came from an era where multivibrator was another name for "oscillator". The word "monostable", in contrast to "astable" and "bistable" is a device with "one stable state".
2. A "trigger" is any signal that is used to start some process. In the case of our one-shot, the trigger will cause the device to change from the one stable state to the other state which is not stable. This does not mean that the not stable state has a value that grows without bound. It just means that the device will not stay in that state for an indefinite amount of time. After being triggered it will change state, then after a predetermined amount of time it will return to its original state, to wait for another trigger.
3. In a two level binary logic scheme there are two voltage levels called "high" and "low". There are also two transitions, one from "low" to "high" and the other from "high" to "low". We call these transitions: "edges". They may be further qualified by saying they are "rising edges" or "falling edges".
Putting this all together, the device you seem to be describing will produce a pulse of some length on either a rising edge or a falling edge. That's a double edge triggered one-shot.

https://www.electronics-tutorials.ws/waveforms/monostable.html
https://www.onsemi.com/pub/Collateral/MC74HC4538A-D.PDF

Joined Jan 15, 2015
7,579
I have a device that switches on with a push button and switches off with another push button.
Would the push buttons be normally open, normally closed or one of each. It's popular to use a N/O PB to latch a circuit ON and a N/C push button to turn the same circuit off (unlatch).

Ron

#### Redge1

Joined Apr 25, 2020
17
Would the push buttons be normally open, normally closed or one of each. It's popular to use a N/O PB to latch a circuit ON and a N/C push button to turn the same circuit off (unlatch).

Ron
Hi Ron they are both NO

#### Redge1

Joined Apr 25, 2020
17
You asked for a circuit and I told you the name of a circuit that I thought you were describing. Google is your friend and "it" will have no trouble providing a wealth of information including dedicated IC's that will perform the function. In the meantime:
1. A one-shot is the colloquial name for a circuit with the official name: Monostable Multivibrator. This somewhat archaic name came from an era where multivibrator was another name for "oscillator". The word "monostable", in contrast to "astable" and "bistable" is a device with "one stable state".
2. A "trigger" is any signal that is used to start some process. In the case of our one-shot, the trigger will cause the device to change from the one stable state to the other state which is not stable. This does not mean that the not stable state has a value that grows without bound. It just means that the device will not stay in that state for an indefinite amount of time. After being triggered it will change state, then after a predetermined amount of time it will return to its original state, to wait for another trigger.
3. In a two level binary logic scheme there are two voltage levels called "high" and "low". There are also two transitions, one from "low" to "high" and the other from "high" to "low". We call these transitions: "edges". They may be further qualified by saying they are "rising edges" or "falling edges".
Putting this all together, the device you seem to be describing will produce a pulse of some length on either a rising edge or a falling edge. That's a double edge triggered one-shot.

https://www.electronics-tutorials.ws/waveforms/monostable.html
https://www.onsemi.com/pub/Collateral/MC74HC4538A-D.PDF
Thank you papabravo, I have been on the internet for the last 2 days looking for answers, I really appreciate people like you taking time to show one in the right direction. I am not a electronics boffin, but it has bitten me because of all the possibilities! I will look at your suggestions.

#### djsfantasi

Joined Apr 11, 2010
9,186
Another term is an “edge detection circuit” This type of circuit produces a short pulse whenever the input goes from low to high (or high to low with some changes).

#### Redge1

Joined Apr 25, 2020
17
I never mentioned, the device I want to switch on and off is a 12v/220v UPS inverter

#### Tonyr1084

Joined Sep 24, 2015
7,991
Post #2 @Papabravo described a component that would achieve the Thread Starter's (TS) desired function. However, one thing I'd be concerned with is whenever the t-stat switches - there is no way of being sure the inverter would act accordingly. It could be possible the t-stat wants to cool the fridge but the inverter might shut off - because the previous state is not guaranteed.

It would seem the TS wants the fridge to continue working via the inverter when a power failure occurs. But the TS doesn't want the inverter working when the fridge is not demanding cooling. So how do we switch the inverter on with the t-stat? I believe that is the appropriate question.

To say the least - this drawing is wrong. The way it's drawn, if the switch is in the wrong position when the t-stat closes - you dead short the utility.

First we need to get a good schematic.

#### Redge1

Joined Apr 25, 2020
17
Post #2 @Papabravo described a component that would achieve the Thread Starter's (TS) desired function. However, one thing I'd be concerned with is whenever the t-stat switches - there is no way of being sure the inverter would act accordingly. It could be possible the t-stat wants to cool the fridge but the inverter might shut off - because the previous state is not guaranteed.

It would seem the TS wants the fridge to continue working via the inverter when a power failure occurs. But the TS doesn't want the inverter working when the fridge is not demanding cooling. So how do we switch the inverter on with the t-stat? I believe that is the appropriate question.

To say the least - this drawing is wrong. The way it's drawn, if the switch is in the wrong position when the t-stat closes - you dead short the utility.
View attachment 207405
First we need to get a good schematic.
Hi Tony, If the toggle switch is in the position as on the diagram, the T switch is just a on off for the 12 circuit. If the toggle is the other way (both levers up) the utility 220v will go through the Fridge motor when the T switch closes. So no shorts

#### Redge1

Joined Apr 25, 2020
17
Post #2 @Papabravo described a component that would achieve the Thread Starter's (TS) desired function. However, one thing I'd be concerned with is whenever the t-stat switches - there is no way of being sure the inverter would act accordingly. It could be possible the t-stat wants to cool the fridge but the inverter might shut off - because the previous state is not guaranteed.

It would seem the TS wants the fridge to continue working via the inverter when a power failure occurs. But the TS doesn't want the inverter working when the fridge is not demanding cooling. So how do we switch the inverter on with the t-stat? I believe that is the appropriate question.

To say the least - this drawing is wrong. The way it's drawn, if the switch is in the wrong position when the t-stat closes - you dead short the utility.
View attachment 207405
First we need to get a good schematic.
O, Sorry, meant to edit the drawing, the switch between 12V and relay 1 is not a switch - just connections

#### djsfantasi

Joined Apr 11, 2010
9,186
View attachment 207403I came up with this circuit, any comments will be welcome.
First, you’re feeding utility power into your 12V supply. Second, your push buttons do absolutely nothing. Nothing at all. I was starting to make a third point, but these two are sufficient to make the circuit a no-go.

#### Tonyr1084

Joined Sep 24, 2015
7,991
My first assumption - and assuming is a dangerous thing - is that a thermostat switch wouldn't be strong enough to withstand the power demands of a fridge compressor motor. However, if - assuming - the t-stat DOES switch the motor directly then when the switch is in the other position the motor is completely isolated from any power.

Even after editing out that superfluous switch by the 12 volt source - there's really no change. The circuit is not supportable. In other words - not useful.

#### Redge1

Joined Apr 25, 2020
17
First, you’re feeding utility power into your 12V supply. Second, your push buttons do absolutely nothing. Nothing at all. I was starting to make a third point, but these two are sufficient to make the circuit a no-go.
Hi djsfantasi, thank you for reply, there is no contact between utility and 12Vsupply with toggle as on the drawing (both levers down) when the toggle is the other way (both levers up) there is only contact between T switch and 12V no contact with utility.
I refer to the edited drawing I am not sure if you saw it yet.

#### Papabravo

Joined Feb 24, 2006
21,304
We often fall victim to the temptation to provide answers on "mere scraps of information".

#### Redge1

Joined Apr 25, 2020
17
My first assumption - and assuming is a dangerous thing - is that a thermostat switch wouldn't be strong enough to withstand the power demands of a fridge compressor motor. However, if - assuming - the t-stat DOES switch the motor directly then when the switch is in the other position the motor is completely isolated from any power.

Even after editing out that superfluous switch by the 12 volt source - there's really no change. The circuit is not supportable. In other words - not useful.
Hi Tony, the fridge is currently switched by the thermostat ( as all fridges are). If you look at the bottem right of the drawing you will see that when the dc circuit provides a mometary switch ON paralel to the ON pushbutton signal on the inverter, the inverter switches on and supplies power to the fridge motor.

#### Tonyr1084

Joined Sep 24, 2015
7,991
The way you drew it - the motor is isolated from any power. I'm missing something. But to re-echo
We often fall victim to the temptation to provide answers on "mere scraps of information".
We just don't have enough (or correct) info.