Ok so I am going to admit that I am an idiot when it comes to circuits. However, some advice would be appreciated with regard to my idea. I am trying to control two seperate circuits, each with different voltage requirements, and hoping to use two relays from one inital push to make switch...so here goes my attempt at the explaination. There is a zener diode in there also, which I need some help with because I may be placing it in the wrong position within the circuit for the outcome that I require.

Background information: The aim of the project is to have two actions take place simoultaniously, with one circuit controlling them both.

Circuit one is using a reference voltage (vehicle ECU) at 5v, this is passed through a POT (throttle pedal position sensor) and then returned to the ECU (0.25-4.75v dependent on POT position). After the POT but before the ECU, there will be a 3.5v zener diode in reverse bais mode, leading to ground, so that the maximum voltage the ECU ever recieves back is 3.5v.

Circuit two is much simpler. It is 12v constant voltage, controlling a solenoid valve, with full open and full closed positions.

Control circuit would be a push to make switch with 12v, the pushing of which button would allow the 12v to pass through to two relays, one of which completing the circuit with the solenoid valve, the other breaking the zener effect on the ECU circuit, and allowing the ECU to recieve more than 3.5v if the POT allows this.

I hope I have explained this enough so as you can understand.

I did have another idea, where the push to make switch would control the initial 3.5v zener only, and then have another 4.5v zener further down the line, so that this 4.5v zener leaked off the voltage to a relay which completed the circuit for the solenoid. This would then only open the solenoid valve when two requirements were met, one being if 4.5v or more was output from the POT, and the other being the push to make switch was activated.

I could be talking complete gibberish, but it kind of makes sence in my head lol

. . .


OK so here is what I have come up with, given your helpful inputs.


simulate this circuit – Schematic created using CircuitLab

As you can see, when the push to break switch is pressed, the voltage the voltage sensor receives is not limited, when the push to break switch is not pressed, the maximum voltage the sensor receives is 3.45v or there abouts.

My next step is to incorporate the solenoid activation in to this some how, so that either:-

A) solenoid is activated when voltage is above 3.45v at the sensor (this would mean the push to break switch must have been pressed) B) much simpler i think, simply using a double pole ( on/on ) switch so that the solenoid is activated, or the zener is activated, never both


There are two variable resistors, both of them (R1 & R2) are connected to the same sensor on the throttle pedal. This is to ensure the ECU can tell if one is faulty, by having voltage linearization between the two, one raising the voltage, one reducing it. Whatever happens to one POT must also happen to the other. SW1 is a push to break switch. The solenoid valve will be wired in after the lamp, I could not find a solenoid sign so I thought why not incorporate a lamp to indicate when this was activated, and also for the purposes of the simulation.

If anyone finds any faults with it, I would be more than grateful if you could bring these to my attention. Thank you.


simulate this circuit

  • 3
    \$\begingroup\$ It would be immeasurably better if, instead of telling us how you propose to get to where you want to go, you just told us your destination. An analogy would be that if you found yourself at the airport of a strange city and needed to get a particular hotel, wandering outside you'd hire a cab and then, instead of telling the driver where you wanted to go, you told him how you thought he should go to get you there. \$\endgroup\$ – EM Fields Dec 25 '15 at 1:02
  • \$\begingroup\$ origin is button not pressed, solenoid valve closed, ecu gets max 3.5v.....destination is button pressed, solenoid valve open, ecu gets max output of the POT \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 1:14
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    \$\begingroup\$ Just summarize and not create a novel dude. \$\endgroup\$ – Andy aka Dec 25 '15 at 1:45
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    \$\begingroup\$ OK, what's a "push to pass" button supposed to make the motor vehicle do? \$\endgroup\$ – EM Fields Dec 25 '15 at 17:38
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    \$\begingroup\$ This whole thing sounds like a lot of dangerous bullshit to me, since if you want to pass someone you have to floor the gas pedal and then take one hand off the wheel to go feeling around looking for a pushbutton switch. Why not just take, say, the last 10% of the gas pedal's travel to kick in the \$N_2O\$ or whatever you're using for the boost and be done with it? \$\endgroup\$ – EM Fields Dec 26 '15 at 0:32

From one of your comments, it seems that all you need is a DPDT (double pole, double throw) pushbutton. One pole would be used to apply power to the solenoid valve, and the other pole would switch the voltage sensor input between the pot wiper and a 3.5 volt source.

  • \$\begingroup\$ the 3.5v is not a separate source of electricity, it is from the POT. Best way i can explain it is that i want the voltage after the POT to be limited to 3.5v max, and there to be a switch deactivating this function, so that the sensor can read the genuine voltage output of the POT and not the limited votage \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 2:52

In response to your last comment (summarizing), as far as I can understand...


simulate this circuit – Schematic created using CircuitLab

Vpot varies from approximately 0.25V to 4.75V.

Vz = Vpot as long as Vpot ≤ 3.5V.

If Vpot > 3.5V, Vz = 3.5V.

When the switch SW1 (DPDT) is flipped (toggling a toggle switch / pushing a pushbutton), your output is disconnected and set low (0V), and your circuit for your solenoid valve is completed (separate from this circuit -- just mechanically connected so they happen at the same time thanks to the switch mechanics).

The only problem I can think of with this circuit is Zener activation current, so you might have to play with these resistor values to get enough current to flow through the Zener Diode at all values of Vpot.

  • \$\begingroup\$ You could also just bias the pot with extra resistors to make Vpot (max) = 3.5V and Vpot (min) = 0.25V. \$\endgroup\$ – Sean Mann Dec 25 '15 at 5:16
  • \$\begingroup\$ I think i realise where i've been going wrong, i've been assuming electricity flows from - to + rather than + to - lol. Yes this is fantastic, thanks. This is what i needed to know. The value at Vz is = to value at Vpot unless Vpot reaches above 3.5v, at which point Vz always = 3.5v. To deactivate this function, could a push to break switch be used after the zener but before ground? \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 12:35
  • \$\begingroup\$ this would work, but i wish to be able to turn off the 3.5v limit when requested. \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 12:41
  • \$\begingroup\$ Just ran a simulation and it works just as I would want, the important part seems to be the 1k resistor after the pot. I assume that using the resistor brings the zener into some sort of operating window? \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 14:10

Your descriptions are very confusing, but I think this will get you where you want to go:

V1 is your 12 volt source, which is used to supply power to L1, your solenoid, and the 78L05, which is a 5 volt series regulator used to convert 12 volts to 5 volts for your pot, comprising R1, R2, and R3, R1 and R3 setting the high and low voltage limits for R2's output. D1 is a catch diode used to protect S1A by clamping the solenoid's turnoff spike to about 13 volts.

S1 is a double pole double throw momentary switch, with S1A being the normally open contacts of one section of the switch, and S1B being the normally closed contacts of the other section.

S1B is used to connect D2 to R2's output, which will allow R2's output to rise to no more than 3.5 volts when S1 isn't being pressed.

When it is, however, S1A will be made, energizing the solenoid, and S1B will be opened, which will allow R2's output voltage to appear, unclamped, at the the input of the voltage sensor.

As an aside, D2 is merely representative of an actual clamp, which could be made from, say, an LM385 or the like.

enter image description here

  • \$\begingroup\$ Please, I haven't got a clue what is going on here. \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 12:42
  • \$\begingroup\$ @JamesShillShillinglaw: Just to allay confusion in the future, I edited out the question in my answer about if you wanted a circuit description, and added the circuit description since you said you wanted one. :) \$\endgroup\$ – EM Fields Dec 25 '15 at 16:10
  • \$\begingroup\$ Thanks, I will read this in a second. I have made a slight amendment to my original question at the top, please have a look and see if the circuit I have shown makes some kind of sense to you, and would it work to give the desired effect. \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 16:24
  • \$\begingroup\$ That seems as though it would work, but the 12v and 5v inputs are seperate. the 12v is purely to operate the solenoid, the 5v purely to provide reference for the sensor after the pot \$\endgroup\$ – James Shill Shillinglaw Dec 25 '15 at 16:44
  • \$\begingroup\$ Did you not read/understand the circuit description you asked for? \$\endgroup\$ – EM Fields Dec 26 '15 at 5:24

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