Really need help on designing a small control circuit

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I have pulled 3 relays out of an old Nissan Maxima. I would like to use the 3 relays on a 12v dc system using 0-5 volt rheostat. The 1st relay needs to turn on around 1.5v, the 2nd at around 3v ( while turning off the 1st one), then the 3rd at 4.5v (while turning off the first 2).

Can anyone help me do this?

Thanks
Rob
 

the following site will provide necessary idea on building the circuits of voltage comparators and switching relays. to turn off previously activated relay
power to all relays are routed through successive higher voltage switching relay NC contacts. (this is simple way to test your comparators later you can do this by electronics)

**broken link removed**
 

You can design a very simple method of operation if the voltages of 1.5//3.0//4.5 are not very exact .
Just use a 1N4148 diode at base of the BC547 used as relay drive .This will go ON at 1.4-1.5VDC .
Use three diodes in series with base for getting the 3VDC transistor switched ON .When the collector voltage of T2 falls ,this drains the base current of T1,which switches OFF..
The same is done for the T3 (third device) which switches OFF the T2 at 4.5VDC
 

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