Question about light sensor circ in Projects section

[johndeverill]

Hi All,

Been looking at the light sensor circuit...

http://www.electronics-lab.com/projects/sensors/001/index.html

and it looks like it would work for a circuit I am creating. Basically I want to replace the LDR with a 0-100mV 4-pin pressure transducer so that at a set pressure, rather than light level, the relay is activated. I would then replace r7 with a larger POT so that the pressure required to activate the relay could be adjusted.

I am so close to working this circuit out I can taste it!!

If anyone can advise on the component swap I would be eternally grateful!

Cheers

John

[ante]

Hi John,

Have you considered this circuit http://www.electronics-lab.com/projects/sensors/004/index.html By changing the 270k resistor to a potentiometer you can vary the size of the

[johndeverill]

Hi Ante,

Thanks for the info. You will have to excuse my ignorance but what is necessary to adapt the voltage divider to my transducer? It has a 10v excitation.....

http://rswww.com/cgi-bin/bv/browse/Module.jsp?BV_SessionID=@@@@1813726976.1101212709@@@@&BV_EngineID=cccjaddddmiedmdcfngcfkmdgkldfil.0&cacheID=ukie&3278665512=3278665512&stockNo=2166253

Many Thanks

John

[ante]

John,

I tried to see it but all I got was this:

[johndeverill]

Hi Ante,

Heres the specs:-

Similar to the compensated versions, these devices differ only in null and full scale outputs. Null exhibits a

[johndeverill]

Hi,

I've had a look at the velleman kit and it appears that the LDR resistance drops and the input goes high on the Opamp which then in turn turns the relay to the off position.

As all I want to do is place the transducer in place of the LDR can do this and then do something such as reverse the polarity of the relay so that it switches on?

Thanks

John

[ante]

Hi John,

The velleman kit has no feedback so in case the pressure changes slowly the relay will rattle. If you want to reverse the relay function you can use the NO contact of the relay or swap the inputs on the opamp.

[johndeverill]

Hi Ante,

Thanks again for the info.

Ill try switching the contacts on the relay. With regards to the relay chatter this shouldn't cause a problem but is it likely to wear the relay out quickly?

I notice the other circuit has a resisitor to prevent this. Can this be done to the circuit I am using?

I do need the relay to snap on as soon as the pressure hits the set value and close again when it drops below. Would adding the resistor slow the response time of the relay?

Thanks

John

[ante]

Hi John,

This extra resistor makes all the difference. The circuit becomes a Schmitt trigger, here is http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/schmitt.html a description of the Schmitt Trigger.