# Help on Relays - Electrical

**URL:** <https://boards.straightdope.com/t/help-on-relays-electrical/122174>\
**Category:** Factual Questions\
**Created:** [August 3, 2002, 2:03pm UTC](https://boards.straightdope.com/t/help-on-relays-electrical/122174 "2002-08-03T14:03:45Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![andy\_fl](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@andy\_fl](https://boards.straightdope.com/u/andy_fl)\
**Post date:** [August 3, 2002, 2:03pm UTC](https://boards.straightdope.com/t/help-on-relays-electrical/122174/1 "2002-08-03T14:03:45Z")

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I need two special types of relays (for lack of a better word) for a project :

1\> Relay #1 - Changes position (i.e. if it was OFF it goes to ON and if it was ON it goes to OFF) when the primary receives a current (I have 110 V ON-OFF Signal - but I can convert that to DC if necessary). It should **NOT** change position when there is no signal. I am trying to illustrate this as follows (t is time in minutes ):

t \< 0, No Primary Signal, Switch is in OFF position  
0\<t\<t1, Primary Signal, Switch is in ON position  
t1\<t\<t2, No Primary Signal, Swith is in **ON** position  
t2\<t\<t2+t1, Primary Signal, Switch is in OFF position  
t2+t1\<t\<t2+t1+t2, No Primary Signal, Switch is in **OFF** position

t1 is expected to be around 1-3 mins. and t2 is expected to be 5-10 mins. The load for the Switch is 110V, 0.5 amps.

Is there any OFF the shelf device which will do this ? Or is there a simple way of making one ?

2\> Relay #2 - This Relay powers up with a delay which can be preset from 1-5 mins.  
Is there any off the shelf devices which will do this ? Or is there a simple way of making one ? I need something which is reliable too.

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**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\
**Post date:** [August 3, 2002, 2:08pm UTC](https://boards.straightdope.com/t/help-on-relays-electrical/122174/2 "2002-08-03T14:08:26Z")

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> [@](#):
>
> \*Originally posted by andy\_fl \*  
> \*\*I need two special types of relays (for lack of a better word) for a project :
> 
> 1\> Relay #1 - Changes position (i.e. if it was OFF it goes to ON and if it was ON it goes to OFF) when the primary receives a current (I have 110 V ON-OFF Signal - but I can convert that to DC if necessary). It should **NOT** change position when there is no signal. I am trying to illustrate this as follows (t is time in minutes ):
> 
> t \< 0, No Primary Signal, Switch is in OFF position  
> 0\<t\<t1, Primary Signal, Switch is in ON position  
> t1\<t\<t2, No Primary Signal, Swith is in **ON** position  
> t2\<t\<t2+t1, Primary Signal, Switch is in OFF position  
> t2+t1\<t\<t2+t1+t2, No Primary Signal, Switch is in **OFF** position
> 
> t1 is expected to be around 1-3 mins. and t2 is expected to be 5-10 mins. The load for the Switch is 110V, 0.5 amps.
> 
> Is there any OFF the shelf device which will do this ? Or is there a simple way of making one ?
> 
> 2\> Relay #2 - This Relay powers up with a delay which can be preset from 1-5 mins.  
> Is there any off the shelf devices which will do this ? Or is there a simple way of making one ? I need something which is reliable too. \*\*

Sournds like you need a latching relay for #1. Solid state time delay units are available. Allied Radio or Radio Shack would be the place to look for both of them.

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**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\
**Post date:** [August 3, 2002, 5:13pm UTC](https://boards.straightdope.com/t/help-on-relays-electrical/122174/3 "2002-08-03T17:13:49Z")

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I would probably do it with a small microcontroller like a PIC controlling a couple of relays via a fairly simply driver circuit. The reason for using the PIC is that you can add some software filtering for relay #1 to make sure it doesn’t glitch and go through multiple ON-OFF-ON-OFF cycles from a single normal command cycle. Also, you can get very accurate variable time delays from a PIC, which would make the control of relay #2 very easy.

Plus, PIC chips are cheap.

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**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\
**Post date:** [August 3, 2002, 11:20pm UTC](https://boards.straightdope.com/t/help-on-relays-electrical/122174/4 "2002-08-03T23:20:04Z")

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A latching relay would be the best. And if you can’t find one of those, you can build a latching relay circuit out of at most 4 standard relays. I can draw up a schematic for you if you want.

I wouldn’t recommend doing it with a MCU, unless you want to buy the nessecary hardware and software and be prepared to learn how to write your own code. For a simple task like this a relay solution will be easier to implement and maintain and more reliable.

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**Author:** ![andy\_fl](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@andy\_fl](https://boards.straightdope.com/u/andy_fl)\
**Post date:** [August 5, 2002, 10:50am UTC](https://boards.straightdope.com/t/help-on-relays-electrical/122174/5 "2002-08-05T10:50:07Z")

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I like the latching relay (thanks David Simmons and Andrew). I am looking at the latching relays from McMaster ([http://www.mcmaster.com/catalog/108/html/0800.html](http://www.mcmaster.com/catalog/108/html/0800.html)).  
However, it says the relays work with momentary signals (the signal i’m looking for will last for like a minute). Will it work with that ?

Also, Andrew can you please give me the schematic?

Thanks again