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simple circuit controlled by a relay without N.O

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ykffc

New Member
I have a motion sensor that has a relay output (I name it RL-A in my circuit).
It has a NC (normally close) output only (but not a N.O. or normally open terminal).

I like to use a DC supply (battery) of 5 volts for powering up a radio when there is motion detected by the sensor.
Because this sensor does not have a N.O output I am thinking I need to build a small circuit like showing below, which I think it should work.
So my load in the circuit is a radio that I already turn on all the time.

Question: what are the values of the resistors, R1, R2 and R3 you reckon?
how about 5K, 1K and 3K ohms?

Of course and please recommend me a "better" circuit as I know I am not a good designer for one.

Thanks for your help.

https://drive.google.com/drive/folders/0By5oEN_IgslpbTFMa3pxS0RrNkk?usp=sharing
 

alec_t

Well-Known Member
Most Helpful Member
That circuit will drop at least 0.7V across the transistor. It would be more efficient to have the radio as the collector load and have the transistor emitter grounded.
It would be even more efficient to use a logic-level MOSFET instead of the BJT shown.
R2 and R3 are not needed.
The base -to- collector resistor value would depend on the peak current your radio draws.
 

dr pepper

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Most Helpful Member
As alec syas.
The resistor depends on the current drawn by the radio, if its a small battery radio then try a 4k7 or a 2k2, of course the transistor must be capable of carrying the current the radio uses.
There will be a waste of power in the resistor as it will be across the battery all the time the sensor isnt activated.
 

ykffc

New Member
As alec says....
if its a small battery radio then try ... ...There will be a waste of power in the resistor ....
1) Yes, will try finding the smallest radio. Actually I just like to hear something rather than watching when and where a possum goes in and out of my house.
Alternatively I try looking for a circuit for a simple 800 hz sounder/buzzer.
2) I have no choice to waste power as I fail to find a working N.O. terminal with my sensor.
 

spec

Well-Known Member
Most Helpful Member
1) Yes, will try finding the smallest radio. Actually I just like to hear something rather than watching when and where a possum goes in and out of my house.
Possum in your house.:happy: Where are you: a state in the USA I would guess?
Alternatively I try looking for a circuit for a simple 800 hz sounder/buzzer.
That would be relatively simple to do.
I have no choice to waste power as I fail to find a working N.O. terminal with my sensor.
In practical terms, you can do what you want with no waste of power in the standby mode.:cool:

spec
 

dr pepper

Well-Known Member
Most Helpful Member
You do have a choice.
Sounds like your using a security PIR, if so it will have a reed relay, you could remove it and replace it with a reed relay that has normally open contacts, or maybe change over contacts, you might need to mount it on flying leads from the pcb if it doesnt fit, and you'll need to be reasonably proficient at soldering.
 

spec

Well-Known Member
Most Helpful Member
Below is a rough sketch for two 'zero power' possum alarm circuits. One circuit energizes a piezo sounder and the other circuit can turn a radio on.

spec

2016_10_25_ETO_POSSUM_ALARM_VER1_iss1.jpg

 
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