Trigger 12V Relay Using Oxygen Sensor Signal (0.2V-0.8V) at 0.5V Threshold

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#1 21673391 12 Feb 2015 19:12

Dale Stieg Dale Stieg

Anonymous

Post #1
21673391 12 Feb 2015 19:12

I'm working on an automotive project and want to take the voltage signal from and oxygen sensor (which puts out .2v -.8v) and trip a relay at .5 volts to drive a small 12v motor (like a windshield wiper motor)

Can I accomplish this without an expensive programmable relay?

Dale
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#2 21673392 14 Feb 2015 18:56

Peter White Peter White

Anonymous

Post #2
21673392 14 Feb 2015 18:56

Take the output of the O2 sensor to a voltage network that increases the 0.5v to about 0.7v and then drive a transistor.

It all depends on the current available from the .2v to 0.8v output
#3 21673393 15 Feb 2015 15:09

Abdur Rehman Abdur Rehman

Anonymous

Post #3
21673393 15 Feb 2015 15:09

If you're going to do more than just tripping a signal - I recommend a PLC (or HMC) solution by Maple Systems http://www.maplesystems.com/ - the 7030 units cost $400...
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#4 21673394 25 Feb 2015 03:34

Frank Bushnell Frank Bushnell

Anonymous

Post #4
21673394 25 Feb 2015 03:34

This can be done with simple electronic circuit, using a comparator IC and a transistor to drive the relay.
#5 21673395 25 Feb 2015 03:39

Peter White Peter White

Anonymous

Post #5
21673395 25 Feb 2015 03:39

You don't even need the comparator. Just put a "set"on the transistor.
#6 21673396 25 Feb 2015 04:39

Frank Bushnell Frank Bushnell

Anonymous

Post #6
21673396 25 Feb 2015 04:39

My idea with the comparator was to give a clean on/off for the benefit of the relay, rather than just a transistor that would gradually turn on as the signal voltage rises.
#7 21673397 25 Feb 2015 04:53

Peter White Peter White

Anonymous

Post #7
21673397 25 Feb 2015 04:53

So what, if the transistor turns on slowly. They relay turns on. We are not making a project for the LUNAR LANDER
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#8 21673398 03 Mar 2015 10:36

Dale Stieg Dale Stieg

Anonymous

Post #8
21673398 03 Mar 2015 10:36

Thanks for the reply. I'm not an electrician. What is a "set" on the transistor.

Can you provide a simple sketch of the circuit that I could put together from components?
#9 21673399 03 Mar 2015 10:38

Dale Stieg Dale Stieg

Anonymous

Post #9
21673399 03 Mar 2015 10:38

Thanks for the reply. I'm not an electrician.

Can you provide a simple sketch of the circuit that I could put together from components?
#10 21673400 03 Mar 2015 11:29

Ruben Proost Ruben Proost

Anonymous

Post #10
21673400 03 Mar 2015 11:29

A germanium transistor only needs like 0.3V
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#11 21673401 03 Mar 2015 15:17

Peter White Peter White

Anonymous

Post #11
21673401 03 Mar 2015 15:17

There are 3 ways to connect the O2 sensor to an amplifier.
You have to know the output impedance of the O2 sensor to be able to decide on which arrangement will work.

You can offset the transistor with a Schottly diode, use a germanium transistor or provide a voltage divider.
#12 21673402 04 Mar 2015 07:45

Jens Grabner Jens Grabner

Anonymous

Post #12
21673402 04 Mar 2015 07:45

The difference between Schottky-Diod and Si-Transistor is ca. 0.35V.
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Topic summary

✨ The discussion addresses triggering a 12V relay using the low voltage output (0.2V to 0.8V) from an automotive oxygen sensor, with a switching threshold around 0.5V to control a small 12V motor. Solutions include using a voltage amplification network to raise the sensor output to a suitable transistor drive voltage, or employing a comparator IC for a clean on/off relay control. Alternatives suggest directly driving a transistor without a comparator, accepting gradual transistor turn-on as sufficient for relay activation. Using germanium transistors or Schottky diodes can help reduce voltage drop and enable switching at lower voltages due to their lower forward voltage compared to silicon transistors. The output impedance of the oxygen sensor is a critical factor in selecting the appropriate interface circuit. For more complex control beyond simple relay triggering, programmable logic controllers (PLCs) such as Maple Systems 7030 units (~$400) are recommended.

FAQ

TL;DR: To trip a 12V relay from an O2 sensor, boost the ~0.5 V threshold to about 0.7 V and "drive a transistor" for switching. [Elektroda, Peter White, post #21673392]

Why it matters: This lets DIYers cleanly trigger a 12V load from a tiny 0.2–0.8 V sensor without a $400 controller.

Quick Facts

Typical narrowband O2 sensor output swings about 0.2–0.8 V; targeting ~0.5 V is common. [Elektroda, Dale Stieg, post #21673391]
Clean switching: a comparator IC plus a transistor can drive an automotive relay. [Elektroda, Frank Bushnell, post #21673394]
Budget check: Maple Systems 7030-class PLCs are about $400, overkill for simple tripping. [Elektroda, Abdur Rehman, post #21673393]
Schottky diode vs Si transistor drop differs by approx. 0.35 V, useful for biasing thresholds. [Elektroda, Jens Grabner, post #21673402]
Germanium transistors can switch around ~0.3 V base‑emitter, easing low‑voltage triggering. [Elektroda, Ruben Proost, post #21673400]

How can I trigger a 12V relay from a 0.2–0.8 V oxygen sensor at ~0.5 V?

Use a comparator to sense ~0.5 V, then a transistor to drive the relay coil. The comparator provides a crisp on/off so the relay doesn’t half‑pull. This keeps the sensor loading minimal and isolates the relay. It’s a simple, low‑cost circuit. [Elektroda, Frank Bushnell, post #21673394]

Do I really need a comparator, or is one transistor enough?

A lone transistor can work, but it may turn on gradually as voltage rises. A comparator gives a clean edge, which relays prefer. "Clean on/off" reduces chatter near the threshold and improves reliability. Choose the comparator route when the switch point matters. [Elektroda, Frank Bushnell, post #21673396]

What did “set on the transistor” mean in the thread?

It refers to biasing the transistor so it switches at your chosen threshold. You can offset with a Schottky diode, use a germanium device, or add a divider. The best choice depends on the sensor’s output impedance and desired trip point. [Elektroda, Peter White, post #21673401]

How do I create a reliable 0.5 V trip point from the O2 sensor?

Scale and offset the sensor voltage so about 0.5 V at the input becomes ~0.7 V to turn a transistor on. That biasing ensures consistent switching and adequate base drive when crossing stoichiometric conditions. Then buffer to the relay. [Elektroda, Peter White, post #21673392]

Why does the O2 sensor’s output impedance matter for this trigger?

Impedance dictates how much you can load the sensor without distorting its reading. It also influences which amplifier or bias network works. Knowing it helps you pick between dividers, diode offsets, or a high‑impedance comparator input. [Elektroda, Peter White, post #21673401]

Can a Schottky diode help me hit the threshold cleanly?

Yes. A Schottky’s lower forward drop lets you set a sharper boundary between off and on. The typical drop difference versus a silicon junction is about 0.35 V, which is significant at sub‑volt signals. Place it to bias your switch point. [Elektroda, Jens Grabner, post #21673402]

Would a germanium transistor make low‑voltage switching easier?

It can. Germanium transistors need around 0.3 V base‑emitter to conduct. That helps when your signal is well under a silicon device’s ~0.7 V knee. Use it if you want simpler biasing near the 0.3–0.5 V region. [Elektroda, Ruben Proost, post #21673400]

Is a PLC or programmable relay necessary for this job?

No. A PLC works but costs far more than a small analog trigger stage. One example discussed costs about $400. For a single threshold and a relay, a comparator plus transistor is simpler, cheaper, and faster to wire. [Elektroda, Abdur Rehman, post #21673393]

Can you outline a simple comparator-to-relay build?

Feed the O2 sensor into a comparator with a ~0.5 V reference.
Take comparator output to a transistor sized for your relay coil current.
Add a flyback diode across the relay and power the relay from 12 V. This yields crisp switching. [Elektroda, Frank Bushnell, post #21673394]

Will a slowly rising sensor voltage damage or confuse the relay?

It won’t damage it; the relay just turns on once the drive is sufficient. If exact timing isn’t critical, a simple transistor path can be acceptable. Use a comparator if you want a defined edge or to avoid hover near the threshold. [Elektroda, Peter White, post #21673397]

What is a comparator IC, and why use one here?

A comparator outputs a digital-level signal when one input exceeds the other. In this project, it converts the small analog swing around 0.5 V into a solid on/off for the relay driver. That minimizes chatter and simplifies tuning. [Elektroda, Frank Bushnell, post #21673394]

How do I prevent relay chatter near the 0.5 V crossing?

Use a comparator stage to produce a decisive transition for the relay driver. This yields a clean on/off instead of a marginal turn‑on. Add small hysteresis if available on your chosen comparator to further stabilize the switch point. [Elektroda, Frank Bushnell, post #21673396]