using a pnp switch and 2 npn switches in serie to control SPDT relay of a 220 motor

Thread Starter

sureshparanjape

Joined Feb 10, 2012
70
I have one pnp and LDR based switch to control a relay for a motor to run in the evening. Two npn based transistor switches, each one is for controlling the same relay if certain conditions are met for two water tanks. One would be driven by output of 555 by two inputs to it from upper tank and other is for the motor to work only if there is enough water in the lower tank. It is desired that relay will get activated (deactivated) when required conditions are met ( any one is not met). These therefore need to be 'in series'. These three are working fine individually on the breadboard. I am unable to connect these to the relay coil in series. Is my information (writing included) enough for an expert to suggest how these be connected so that if any requirement is not met the motor wouldn't 'run' ? Many thanks in advance to helping nature Samaritans on the forum.
S R Paranjape
 

Thread Starter

sureshparanjape

Joined Feb 10, 2012
70
I think I got my error.
Since the 'sensors' were operated from the main source, the presence of small current at NPN transistors' bases was making led glow. I was inferring that my circuit is somehow improper. The second figure shows correct(?) way to connect them.
I will go back to breadboard and try out.
I am sorry for this slip in my thinking and troubling you nice people.
I am giving outlines of the circuits(with error and corrected),since (it appears) three small circuits working as desired. Thanking you all.S.R.ParanjapeRightCircuit.png inputs for operation of relay.
 

Alec_t

Joined Sep 17, 2013
10,907
One important thing missing from those circuits is a reverse-biased diode across the relay coil to suppress back-emf spikes when the relay is switched off.
 

Reloadron

Joined Jan 15, 2015
5,122
This looks like you want a 3 Input permissive loop circuit. Three conditions need to be met for something to happen. Rather than using 3 transistors would it be practical to use, for example a Three Input AND Gate? A chip like the DM74LS11 or similar. When all three conditions are met (Logic High) a single transistor is turned on for your relay.

Here is a rough example:
Permissive Loop.png

Any of several gates could be used the 7411 is just an example of a 3 input AND gate. Sens1, Sens2 and Sens3 all need to be High for the output to go high and turn on Q1. Additionally a LED could be added easily indicating all three signals were present.

Ron
 

AnalogKid

Joined Aug 1, 2013
8,306
Here is a re-draw of your schematic, showing what I think you want. This schematic has a protection diode across the relay coil, andseoarates the LED from the coil. In this way, the relay coil current does not flow through the LED. This lets you adjust the LED brightness withR1 without affecting the operating voltage and current of the relay.

Ignore the dashed lines, they are caused by the PDF conversion program.

ak
3-Switch-Relay-1-c.gif
 

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Thread Starter

sureshparanjape

Joined Feb 10, 2012
70
One important thing missing from those circuits is a reverse-biased diode across the relay coil to suppress back-emf spikes when the relay is switched off.
I have included it in my detailed circuit. However thank you for pointing the omission.
I haven't still put it on breadboard. Relay is not getting connected on breadboard- the "legs"
are rather wide.
 

Thread Starter

sureshparanjape

Joined Feb 10, 2012
70
Here is a re-draw of your schematic, showing what I think you want. This schematic has a protection diode across the relay coil, andseoarates the LED from the coil. In this way, the relay coil current does not flow through the LED. This lets you adjust the LED brightness withR1 without affecting the operating voltage and current of the relay.

Ignore the dashed lines, they are caused by the PDF conversion program.

ak
View attachment 122945
Many thanks.
I couldn't have imagine the right way of adding led, parallel to protection diode!
My favorite quote that I have come across: Problem is difficult till you see the solution.
After seeing the solution you wonder how come I didn't think of that!
I am using BC 547 and BC557(PNP). Will it cause problems for circuit?
Thanks again for this kind help.
 
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