# Relay to switch between two low voltage (1v or less) signals

Joined Feb 12, 2016
4
Hi, I'm completing a project on a car involving oxygen sensors; which use a chemical element that reacts to the amount of air present to generate a small voltage that is sent to the car's ECU. The car's computer only has one input channel for one o2 sensor, but I'm adding a second o2 sensor. I know a little bit about relays, and I'm wondering what kind of circuit could take the small voltage output from two separate sensors, and switch between them a few times per second to output to the car's computer? The circuit would have to have minimal voltage loss since the voltage is already so low. Any help is greatly appreciated here.

#### ScottWang

Joined Aug 23, 2012
7,411
1. What voltages you want to use(as 12V)?
2. Probably use relay is not a good idea, since it working a few times per second, maybe you could use CD4066.

Joined Feb 12, 2016
4
1. What voltages you want to use(as 12V)?
2. Probably use relay is not a good idea, since it working a few times per second, maybe you could use CD4066.
I'm sorry I'm not sure I understand; do you mean as the power source for the circuit? If so then yes 12v would be ideal.

#### ScottWang

Joined Aug 23, 2012
7,411
I'm sorry I'm not sure I understand; do you mean as the power source for the circuit? If so then yes 12v would be ideal.
Sorry, I forgot to say the voltages providing for the circuit, but this is easy to guess, and the CD4066 is easy to use, the In/Out pin and Out/In pin just like the conducts of relay, and the cont is the control pin, you just give it +12V and the In/Out pin and Out/In pin will be as a switch, but it has a on-state resistance about 80Ω(I measured it), I tested it before and it had 0.1% tolerance, it means that the voltages through In and Out pins will be have a 0.1% tolerance.

Joined Feb 12, 2016
4
Sorry, I forgot to say the voltages providing for the circuit, but this is easy to guess, and the CD4066 is easy to use, the In/Out pin and Out/In pin just like the conducts of relay, and the cont is the control pin, you just give it +12V and the In/Out pin and Out/In pin will be as a switch, but it has a on-state resistance about 80Ω(I measured it), I tested it before and it had 0.1% tolerance, it means that the voltages through In and Out pins will be have a 0.1% tolerance.
Thank you very much Mr. Wang. So if I understand correctly, I can hook up both the o2 inputs and this will switch between them for the single output? Is there any programming involved? I must admit I have a very limited understanding of circuitry; my forte is more mechanical.

#### ScottWang

Joined Aug 23, 2012
7,411
Using the real switch Sw1 to select COM,S1 and COM,S2
COM -- This is the common pin of S1 and S2
The Switch 1 -- COM and S1
The Switch 2 -- COM and S2
Using Control S1(+12V) to turn on Switch 1
Using Control S2(+12V) to turn on Switch 2
Connected to GND for Useless gates as in/out pin and control pin.
No any program involved, you just need to wiring them.
The circuit was designed for fixed switch when you switching on one side.

#### ScottWang

Joined Aug 23, 2012
7,411
The circuit was designed to switching several times for one second.

The circuit was designed to using one push switch to switching two digital switches.
Another unuse flip flop of CD4013, the input pins should be connected to Gnd as below, and Pin 14 connecting to +12V, pin 7 connected to Gnd.

#### slackguy

Joined Feb 11, 2016
76
you would have to amplify the signal and have high frequency able parts

the O2 signals can be measures as "0V cold 1V operating" for rough diagnosis

but the car computer needs to see voltage variances in the O2 sensor that you'd need a good oscilloscope to see: they are very fast switching and average 1V to a rough measurement. the fast switching is read by a sensitive comuter for levels per cyl fire (so, wave signal of O2 is > 2kHz)

do NOT toy with the emmisions (O2) unless required. it's probably illegal. operating an engine that has damaged emissions parts is illegal. it's not just efficiency or potental "smog". it's also a law because people who run with poor emmisions destroy the engine. clean and simple: they forbid you from wasting a good engine

here's an example from a dealer mechanic i heard: always they got business from local shops who messed things up, also people doing mods. on is the "muffler mod" that "adds %10 efficiency". sure it did: but it also, after a couple years, DESTROYED the engine (some engines react very poorly, gum up, etc if emmisions components are not working). the guy wanted a new motor on warrantee. the dealer told him: no. modifications are not covered.

Joined Feb 12, 2016
4
So this circuit will complete the switching automatically? I see there is a component called "push switch". Once again, I'm sorry; I'm still very new to all of this.

#### ScottWang

Joined Aug 23, 2012
7,411
So this circuit will complete the switching automatically? I see there is a component called "push switch". Once again, I'm sorry; I'm still very new to all of this.
The last one circuit in #7, when the power up, the pin 1 (Q1) of CD4013 will be low(0V), and the control S2 will be low(0V) too, SWB will be turn off , so the S2 is turn off, the pin 2 (/Q1) of CD4013 will be high(5V), and the control S1 will be high(5V) too, SWA will be turn on, so the S1 is turn on.

The out of CD4013 active as a toggle switch controller, when you push the push sw1 once then it will be active toggle once and the output of CD4013 will be changed, pin 1 (Q1) will be turn to high(5V) and pin 2 (/Q1) will be turn to low(0V), and the S1 turn off, the S2 turn on, if you push the push sw1 again then it will be change the output status again and the S2, S1 will be change again.

#### AnalogKid

Joined Aug 1, 2013
11,128
I disagree with the choice of a 4066 for this application.

1. Its series resistance in the ON state is highly dependent on temperature. While the typical value is 180 ohms at +25C, the max value is 500 ohms at +85C, a temperature that is very easy to reach under the hood of a car.

2. Its series resistance varies greatly with voltage. That is, as the signal going through the switch varies as it is supposed to, the resistance the switch puts into the signal line varies.

3. Even if the 180 ohm value were constant, that is a very large resistor to place in a system designed around a zero-ohm piece of copper wire. Depending on the output impedance of the sensor and the input impedance of the controller, this could wipe out the signal or significantly change its perceived frequency characteristics.

ak

#### Kermit2

Joined Feb 5, 2010
4,162
You really need another micro to do the switching and a quality buffer op amp to unload the inputs and output.
Relays are terrible awful bad no good for digital data signal switching.
Where are the pic people? Where is my pickit kit? I need to dig that baby out and make the blink light blink.
Sorry, but I know what will work in this case but haven't used it myself so I cannot do more than say you need it, but someone else will have to help you build it.