simulate this circuit – Schematic created using CircuitLab

Figure 1. Original Schematic

Is there any way to get rid of the relay in this circuit?
And would it be wise to add in a power regulator? If so how?


simulate this circuit

Figure 2. Edited @Transistor's schematic

  • Changed Transistors to PNP
  • Changed Bottom Proxy to NO 2 wire proxy
  • Changed Top Proxy to NO 2 wire proxy
  • Added 2nd Digital Counter signal


simulate this circuit

Figure 3. Choke and Capacitor Filter

If I did my research correct this should do the trick... Or is my research wrong.? @Transistor


  • 1
    \$\begingroup\$ Your question needs a lot of work. What is the intent/function of the design? What are the power source(s)? If TR1 is a TRIAC, that usually is used to control AC - yet you have a low-voltage, low-current signal diode across a relay coil driven by the TRIAC There are two things labeled "PNP which I suspect are not transistors. The answer to your question probably is yes, but who can say why? \$\endgroup\$ – AnalogKid Mar 26 at 19:55
  • \$\begingroup\$ This does not look good without datasheets, specs for all power sources and loads and functional response time, power levels,EMI etc \$\endgroup\$ – Sunnyskyguy EE75 Mar 26 at 20:17
  • \$\begingroup\$ The proper way to design anything here is to define all the Inputs, processes and outputs with power , surge loads and time responses and required power functions. Asking to get rid of Relay is certainly possible but even a switch has specs for V,I contact bounce time and arc current rating. the motor DCR and stored energy to be switched must be defined. Motors or solenoids have stored energy that must be dissipated by design. \$\endgroup\$ – Sunnyskyguy EE75 Mar 26 at 20:26
  • \$\begingroup\$ Input voltage comes from a 12v car battery... The "things" labeled with PNP are proximity sensors... The Function of this Circuit is to automatically flip flop power to the up and down solenoids. The sensors are attached to a hydraulic cylinder and the solenoids to a valve. (With the idea that the hydraulic cylinder would go up and down automatically) As far as the all the specific specs... I don't have them... I'm trying to figure out the theory of the design first then size all the components after. \$\endgroup\$ – Sam burris Mar 26 at 21:10
  • \$\begingroup\$ Why don't you like the relay? \$\endgroup\$ – yhyrcanus Mar 26 at 21:23

First draw a timing diagram. I'll use NO switches for both sensors.

          _                          _
Up Sw ___| |________________________| |_______
Dn Sw ________________| |_____________________
       ___              _____________
Up SV     |____________|             |________
           ____________               ________
Dn SV  ___|            |_____________|

Then draw the circuit schematic.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Modified circuit.

Hot it works:

  • Hitting the top sensor triggers the SET input on the latch. This turns on Q1 and powers the Down SV (solenoid valve).
  • As the actuator moves away from the top prox sensor it turns off but the latch maintains the motion.
  • On reaching the bottom sensor the RESET input is activated and the Up SV is turned on reversing the motion. If an NO sensor is used here NOT1 can be omitted.


Could you get away with a 2 wire proxy sensor...

Yes. But these have leakage current as the sensor requires power even when off. For that you need a pull-down resistor. The resistor value needs to be low enough that the input voltage to the latch is low enough to ensure that you always get a logic zero on the input when the switch is off.

Also is there any reason that the Up/Down SV's are in a NPN config.? As this may be an issue with the signal going to the Digital Counter.

It's the normal way of doing things, often because the logic is 3.3 V or 5 V and the load is higher voltage. (See High-side fail for an explanation.) What you have sketched should work OK.

D9 is doing nothing so you can leave it out. D14 is a bad idea as it gives you two different ground voltages. In any case, it's redundant if you have D9 in.


simulate this circuit

Figure 2. Updated schematic.

A couple more items:

  • CMOS logic can be run at > 12 V so that would be a good choice for the latch.
  • Regarding your regulator question. It might be a good idea to add one to prevent vehicle voltage spikes reaching your logic but this will reduce the voltage to below 12 V so now you have to limit the voltage from your sensors and you're back to NPN switches for the solenoids. Some local filtering - a choke and capacitor - may suffice.

... this may be an issue with the signal going to the Digital Counter.

No. It will be looking for low-high transistions. It won't know the difference.

  • \$\begingroup\$ Thanks for the pointing in the right direction... I changed the transistors in your schematic to PNP. Will the way I have drawn in the edited schematic work as well? Thanks -Sam \$\endgroup\$ – Sam burris Mar 27 at 19:32
  • \$\begingroup\$ NPN now makes sense to why it's the norm...D9 Was for if anyone hooked the battery up backwards or is this not an issue? Also You say a "choke and capacitor" are you taking about an upstream resistor and capacitor? @Transistor \$\endgroup\$ – Sam burris Mar 27 at 19:58
  • \$\begingroup\$ Yes. Feed the whole circuit through the choke with the capacitor on the circuit board side of the choke. I can't help you with the design of that but you should be able to find help online. \$\endgroup\$ – Transistor Mar 27 at 20:02

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