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Fire alarm using a PTC as a sensor.

m_b_anf

Jan 6, 2023
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I have been trying to do a circuit with the following charcteristics:

The output is a siren connected to a 230 V 50 Hz network. The control circuit will use a 9 V battery as power supply and an operational amplifier must be used, the siren will be activated with a relay. The alarm will be activated when the temperature is higher than 60 degrees celsius. (The simulation was made with simulide)

I also used a BJT transistor to control the passing of the current, I think the control circuit and input circuit is correct, somehow the relay doesn't close and so the led doesn't switch on. What is wrong with my circuit?

circuit_alarm_ptc.PNG

Moderators note : expanded schematic.
 
Last edited by a moderator:

Alec_t

Jul 7, 2015
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Check the switch in the relay. Do you see a problem there?
What do you think the comparator output will be if the PTC resistance increases?
 

bertus

Moderator
Nov 8, 2019
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Hello,

Check your schematic.
What is the type number for the comparator?
Does the comparator need a pull-up resistor?
How is the relais powered?
Can the bridge work, the way you have drawn it?

Bertus
 

AnalogKid

Jun 10, 2015
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There is no common ground. The input circuit, comparator, and output circuit all must be referenced in some way to the same DC potential. They do not necessarily have to be hard wired together - there are some instances where you might want a DC offset between the input and output.

But in your case, they must be hard-wired together.

If you add a unique reference designator to each component, the exact connections can be described. Without reference designators, you're on your own.

ak
 

danadak

Feb 19, 2021
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Feb 19, 2021
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Also output of comparator should have a series R to the transistor to limit
base current. Current should be limited to ~ Relay coil current / 10. Yiou
should also have a snubber or diode clamp to manage L based transients
from damaging the transistor.

1673090404455.png

R2 should be ~ 10 x R1, R1, depending on relay coil current needed, 100 - 1000
ohms. Ibase = Ic / 10 to help you compute it.


Regards, Dana.
 

davenn

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Sep 5, 2009
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What is wrong with my circuit?

lots :) :)

Dana has given you the correct info regarding the transistor and relay wiring.
The way you have wired it, the relay will never operate as it isnt getting any power.
Also, even with that fixed, the LED will not light as it doesnt have a complete power supply

In reality, if the only thing you are operating is the LED, then the relay isnt needed and the LED and
its series 1k resistor can be put in place instead of the relay and it's snubber diode

EDIT, one moment you are talking about a siren ""output is a siren connected to a 230 V 50 Hz ""
Then you talk about a LED
So which are you really using ?
If a 240V siren, then a relay will be needed and it's switching contacts need to be capable of
240VAC and whatever current the siren unit requires ??


cheers
Dave
 
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