Change trigger from NO to NC

[Heckerpe]

I have a **broken link removed** Select-Alert alarm. The alarm trigger is NO and is activated when the circuit is closed. I need to reverse that. I will be using a NC loop as the trigger so if the loop is cut the alarm will be activated which is the reverse of how it operates now. The only thing I can figure out is to use a relay. Any other suggestions and/or how to wire in a relay? Here are all the specs that I can find. TIA **broken link removed**

 

[Les Jones]

Hi Heckerpe,
It is probably possible to do what you want using a small transistor or mosfet and a few resistors. First you need to find out if the input connection of the contacts switch to supply negative or to some positive voltage. Start by measuring the voltage on the contact connections (We will call the connections L & R - For left and right.) with respect to the negative supply both with the contacts open and closed. Measure the current that passes through the contacts. Also check if the battery negative is connected to the 12/24 volt supply negative. With this information it should be possible to work out how to achieve what you want. Will you be powering the unit from the battery or an external supply. (This is to decide how much we need to reduce the current consumption of the circuit.

Les.

 

[Tony Stewart]

With Dry Contacts "1FormC" in product spec, this means SPDT
Yet they have stated Alarm = Closed contact.

NO contact pair logic can be inverted using a pull down resistor to gnd and connect other side to Vcc.

Although to detect cut lines requires 3 wires to remote sensor and remote pull down R and a higher R to pull up locally if all lines cut.

Vcc : N.O. pin 1
Input: N.O. pin 2 // R ( pull down)
0V return : R (pull down)

 

[Heckerpe]

Thank you for the fast response. Most of my electronics training and memory have long been lost so please bear with me as I try and recover it thru out this project. This is what info I have in response to your questions. I know it doesn't answer them all but hopefully it is a start.

 

[Tony Stewart]

OK then you already have everything.
You supply remote power +,- and jumper both MAIN and ACTIVE unless needed.
Then floating contacts out with COM and NC and connect as a high side or low side as you require.


Next define what the alarm drives.

 

[Heckerpe]

With Dry Contacts "1FormC" in product spec, this means SPDT
Yet they have stated Alarm = Closed contact.

NO contact pair logic can be inverted using a pull down resistor to gnd and connect other side to Vcc.

Although to detect cut lines requires 3 wires to remote sensor and remote pull down R and a higher R to pull up locally if all lines cut.

Vcc : N.O. pin 1
Input: N.O. pin 2 // R ( pull down)
0V return : R (pull down)

In reading the specs and somewhere else the "Dry contacts" are used for daisy chaining a remote alarm to this one whereby if the remote alarm sounds it will also trigger this alarm.

 

[Heckerpe]

OK then you already have everything.
You supply remote power +,- and jumper both MAIN and ACTIVE unless needed.
Then floating contacts out with COM and NC and connect as a high side or low side as you require.


Next define what the alarm drives.

LEDs and a piezo.

 

[Tony Stewart]

A 2 wire remote switch is not fool proof. Wires can either be cut or shorted. If that just a marginal security requirement?

 

[Heckerpe]

A 2 wire remote switch is not fool proof. Wires can either be cut or shorted. If that just a marginal security requirement?

It works for my purpose. The alarm is used to tether, via the loop, items I display at swap meets. I've got 32 - 40 feet of tables to watch over. I'll be at one end with a customer and something walks away from the other end.

 

[Les Jones]

Hi Heckerpe,
I suspect something like this should enable the unit to work with a normally closed loop.

You will need to answer the unanswered questions from post #2. (What is the voltage on the non grounded terminal on the existing open loop input and what is the current between the two alarm input terminals.) When you answer these questions I will suggest resistor values and suggest some suitable transistors that you can use. R1 and C1 only provide some protection from RF interference being picked up by the loop and possibly causing false triggering.

Les.

 

[Heckerpe]

OK then you already have everything.
You supply remote power +,- and jumper both MAIN and ACTIVE unless needed.
Then floating contacts out with COM and NC and connect as a high side or low side as you require.


Tried...If I jumper the "alarm switch" terminals the alarm triggers. The COM and NC and NO terminals do nothing. I think they are looking for a trigger voltage from a remote alarm. The only way I have been able to get the alarm to trigger is using a jumper to "connect" rather than the "disconnect" I am looking for.

 

[Heckerpe]

Hi Heckerpe,
I suspect something like this should enable the unit to work with a normally closed loop.
View attachment 93387
You will need to answer the unanswered questions from post #2. (What is the voltage on the non grounded terminal on the existing open loop input and what is the current between the two alarm input terminals.) When you answer these questions I will suggest resistor values and suggest some suitable transistors that you can use. R1 and C1 only provide some protection from RF interference being picked up by the loop and possibly causing false triggering.

Les.

I'll get back to you. Need to clear my mind........

 

[Heckerpe]

You will need to answer the unanswered questions from post #2. (What is the voltage on the non grounded terminal on the existing open loop input and what is the current between the two alarm input terminals.) When you answer these questions I will suggest resistor values and suggest some suitable transistors that you can use. R1 and C1 only provide some protection from RF interference being picked up by the loop and possibly causing false triggering.

Les.[/QUOTE]

I get 5VDC at the non-ground alarm switch terminal. I forget how to interpret current readings ..... but my meter is set to 20m and the reading is 0.50

 

[Les Jones]

Hi ToddB74,
Make R1 100 ohms, R2, 15K, R3 150K, C1 100nF. Almost any small NPN transistor should work. (2N3704, 2N3904, BC337) You could also use a small mosfet such as an 2N7000. (Source would connect in place of emitter, gate would connect in place of base and drain in place of collector.) You could leave R3 out as it just limits the base current to about 50 uA which is more than enough to saturate the transistor. I am assuming the current you measured is 0.5 mA so I have set the current in the alarm loop to be about this value.

Les.

 

[Heckerpe]

Hi ToddB74,
Make R1 100 ohms, R2, 15K, R3 150K, C1 100nF. Almost any small NPN transistor should work. (2N3704, 2N3904, BC337) You could also use a small mosfet such as an 2N7000. (Source would connect in place of emitter, gate would connect in place of base and drain in place of collector.) You could leave R3 out as it just limits the base current to about 50 uA which is more than enough to saturate the transistor. I am assuming the current you measured is 0.5 mA so I have set the current in the alarm loop to be about this value.

Les.

Thank you Les. Will give it a try and let you know.

Best regards,
Perry

 

[Heckerpe]

Thank you Les. Will give it a try and let you know.

Best regards,
Perry

Hi Les,
I got the parts in yesterday but didn't test until this morning. BINGO!!!!!!! We have a winner!!! At first it wouldn't trigger so I re-read your specs again (20th time) and replaced R3 with a jumper. Tested perfect. I thought maybe I misaligned a pin so I replaced R3 but no-go. Back to the jumper and BINGO again! Thank you so much. I have spent too many hours thinking about it and should of posted here weeks ago.

Very best regards,
Perry

 

[Les Jones]

I'm pleased that it solved your problem. The reason it does not work with R3 in could be the hfe is lower than the value I used for my calculation or the current that you measured was more than the 0.5 mA I thought it was. The only effect of not having it is the circuit will take about 450 uA more when the alarm loop is open. As this situation will only occur for a very short time it will have almost no effect on the battery life.

Les.

 

[Heckerpe]

I'm pleased that it solved your problem. The reason it does not work with R3 in could be the hfe is lower than the value I used for my calculation or the current that you measured was more than the 0.5 mA I thought it was. The only effect of not having it is the circuit will take about 450 uA more when the alarm loop is open. As this situation will only occur for a very short time it will have almost no effect on the battery life.

Les.

We'll just blame it on the new guy....me.