Relay alternative.

Other Hardware General Electronics
1

I am using currently using a bulky 8 pack relay board to control the climate control on my truck. All they relays do are short two wires together that have virtually no current running through them. Is there a less bulky solution that I can use that is cheap? I was thinking about using transistors but I think that the climate control reads button presses by resistance so I don't know if they will work.

Essentially I want a solution other then a large relay board to short two wires together while being isolated.

2

Based on your post, it is evident that you have no electronics experience.
ie:

All they relays do are short two wires together that have virtually no current running through them. Is there a less bulky solution that I can use that is cheap? I was thinking about using transistors but I think that the climate control reads button presses by resistance so I don't know if they will work.

Based on your post, it is evident that you have no electronics experience.
ie:

Well , guess what , That makes absolutely no sense at all. Those relays have 3 contacts, (COM,N.O.,N.C), just like any other relay, and the amount of current that passes through those contacts has absolutely NOTHING to do with the relay design. Load current is set by the load. You have not stated what your "load" is and your above statement is complete nonsense.
Please rephrase whatever it is you were trying to say.

Please start your post and state the reason you need a relay alternative. What exactly are you trying to do ? "Exactly" means just that. Please don't omit any details. There are many alternatives but in order to choose one we need to know the whole picture.

3

[u]Here[/u] is a small solid state relay.

If the current is low enough, you may be able to use an [u]analog switch[/u] which can save a bit more space.

Essentially I want a solution other then a large relay board to short two wires together while being isolated.

If you don't know anything about the circuitry, a relay is best because a relay is an electrically isolated switch so it can replace a regular mechanical switch. A transistor or MOSFET isn't isolated so it can affect your circuit in unpredictable ways, and transistors/MSOFETs are not bidirectional so you have to get the voltage & current in the correct direction.

You have to be a bit more careful when selecting solid state relays (than when selecting an electromechanical relay) because some can only switch AC and some don't work at low voltages, etc.

4

If you don't know anything about the circuitry, a relay is best because a relay is an electrically isolated switch so it can replace a regular mechanical switch. A transistor or MOSFET isn't isolated so it can affect your circuit in unpredictable ways, and transistors/MSOFETs are not bidirectional so you have to get the voltage & current in the correct direction.

You have to be a bit more careful when selecting solid state relays (than when selecting an electromechanical relay) because some can only switch AC and some don't work at low voltages, etc.

I couldn't agree with you more but unfortunately I think you are speaking Greek as far as the OP is concerned. Anyone who could ask the question he asked could not have a clue what you just said.

5

raschemmel:
I couldn't agree with you more but unfortunately I think you are speaking Greek as far as the OP is concerned. Anyone who could ask the question he asked could not have a clue what you just said.

Dude... you are coming off as quite rude.

Anyways... Here is my question rephrased in better English.

Here is the circuit board from the back of my ATC unit.

The wires that are soldered to the circuit traces are used to simulate button presses such as adjusting fan speed or temperature. For example, shorting wire 1+2 might decrease temperature while shorting wire 3+2 might enable defrost.

Currently I have the 8 relay block shown in my first post acting as the means to short the proper wires together to activate certain functions. It is my understanding that chip on the ATC board recognizes which button is pressed by reading its resistance. I have a rudimentary understanding of electronics as I went to tech school for instrumentation engineering but I only had a couple of beginner electronics classes.

Now I may be incorrect but I think the circuit has to be isolated as it reads resistance and I don't think I should introduce any extra current or voltage to it.

My question is if there are any other options that I would be able to use other then a regular relay.

6

If there is a DC voltage on the switches, e.g. they switch 5volt to ground, you could use opto couplers.
You will need an additional current limiting resistor for the opto LED.
Leo..

7

Dude... you are coming off as quite rude.

Yes, I get that a lot. I call it like I see it. I think you should swallow your pride and man up by starting your post by saying :
"I don't know jack about electronics and I need help hacking a circuit board ."

It would save us a lot of time. If I'm wrong , then tell me which one of DvDDoug's suggestions makes more sense .What is it he told you that is the most important thing you need to know ?

shorting switches with transistors

optocoupler

@Wawa,
You beat me by 6 minutes because my post took so long..ha ha.

The optocoupler requires additional components on both input and output. This ic is not an option until you have first done what DvDDoug recommended, which is get a DMM and measure the voltages across the contacts or the two points you want to short. It is a dc circuit so we have to start by establishing polarity. It is not certain yet whether the internal resistance of the optocoupler output transistor is an issue yet. Micro relays (they can get very small) is the ideal solution because as DVDDOug pointed out, they are idiot proof. (that means even you can use them).

Get the measurements , draw a diagram , do you know what a schematic is or how to draw one ?
if not , draw a block diagram and write the voltages across the two points you measure . Put the BLK
meter lead at the point that results in a POSITIVE voltage and then label the two points "+" and "-" with the voltage measured across them. We will tell you what to do after you post that info.

The 8-pin DIP relay is the best option, IMNotsoHumbleO

8

DVDdoug:
[u]Here[/u] is a small solid state relay.

If the current is low enough, you may be able to use an [u]analog switch[/u] which can save a bit more space.
If you don't know anything about the circuitry, a relay is best because a relay is an electrically isolated switch so it can replace a regular mechanical switch. A transistor or MOSFET isn't isolated so it can affect your circuit in unpredictable ways, and transistors/MSOFETs are not bidirectional so you have to get the voltage & current in the correct direction.

You have to be a bit more careful when selecting solid state relays (than when selecting an electromechanical relay) because some can only switch AC and some don't work at low voltages, etc.

I just looked into FETS/Analog switches and it looks like they will add to much resistance when closed. Mechanical relays seem to be the way to go in this case.

raschemmel:
The 8-pin DIP relay is the best option, IMNotsoHumbleO

I looked at this datasheet http://www.everlight.com/file/ProductFile/8Pin_DIP_SSR.pdf and it looks like they typically add 20 ohms which I fear will be too much.

raschemmel:
Get the measurements , draw a diagram , do you know what a schematic is or how to draw one ?
if not , draw a block diagram and write the voltages across the two points you measure . Put the BLK
meter lead at the point that results in a POSITIVE voltage and then label the two points "+" and "-" with the voltage measured across them. We will tell you what to do after you post that info.

I cannot measure the voltage right now as it requires pulling out the whole unit and I need the heater to defrost the windows in the morning.

9

MrSHME:
I just looked into FETS/Analog switches and it looks like they will add to much resistance when closed.

Actually, they will do just fine! What you illustrate are typical "chiclet" type switches with a carbon contact whose resistance is quite significant, so anything less than 100 Ohms is likely to do perfectly well.

MrSHME:
I cannot measure the voltage right now as it requires pulling out the whole unit and I need the heater to defrost the windows in the morning.

Well, you do need to know the voltages as it determines the analog switch series that you will be able to use. If it is five volts, then the 74HC4066 provides four independent control switches with an "on" resistance of 50 Ohms. If they have a common connection a 74HC4051 can control any of eight switches with less than 100 Ohms "on" resistance.

So go check the voltages - if it is more than 5V, a slightly different circuit will be required. You can of course, also verify that a 100 Ohm resistor bridged across the relay contacts has the same effect as a dead short. I should be very surprised if it did not

Of course, you only need to get at your present relay board to measure the voltages, not the main board. But you do want to minimise the length of your interconnect wires to avoid interference problems in a car.

10

Use small mechanical relays.

11

Small 5V reed relays have worked for me to simulate "pressed button".

They can't handle very much current (they'll burn or weld shut), but for this they will work fine.

They are inductive, so you should use a fly-back diode to avoid frying the Arduino pins.

12

That's what I'm talking about.

13

Robbie the robot is sometimes helpful. In 8000 posts with a similar level of rudeness, we haven't kicked him out yet.

All of the suggestions above deserve consideration. Most of them will work. I doubt any of them will make the project smaller or simpler for the OP.

My suggestion is really a question: "Can you share ground with the AC unit controls?" If you can find the 0V line on the board and you can connect your Arduino ground to it, then you are halfway done. Most likely it shares ground with the rest of the vehicle, so that should not be difficult.

Then measure the voltage on the switch pads. Hopefully one is ground and one is +5V or +3V when the switch is not pressed. This second pad can be connected directly to an Arduino digital pin. digitalWrite(PinName, LOW); will 'press' the switch. (If it's 3V and you're using a 5V Arduino then you never want to write HIGH to that pad - set the Arduino pin as an input to make it un-pressed.)

Note: I would not be making this suggestion if the OP hadn't already told us that he knows which end of the multimeter to hold.

14

If the OP is able to access the board to take the measurements I still think optocouplers is an option. Analog switch may still be viable. I think the OP has made many assumptions with nothing to support them. He should be trying all the suggested options and keeping his opinion to himself. If he knew what to do he wouldn't be posting here , would he ?

In 8000 posts with a similar level of rudeness,

Just for the record, it's 9200 posts, and I'm not always rude. That is always based on something the OP has said or NOT said. One of my pet peeves is people who don't have the guts to come out and admit they don't know something. Our job would be so much easier if everyone was honest enough to say they don't know jack about electronics. At the very least they should begin their post with an explanation of the extent of their electronics experience. (ie: "I've done a few projects..." or " I have no hands on experience." Posting here asking for help hacking a circuit board without disclosing experience or lack thereof is a waste of everyone's time. This OP has still not come clean about his experience. If he at least did that I would be more amenable (I'm not going to say "friendly" because we have be realistic here....)

15

Thanks for all the great responses guys.

@rashemmel I agree with most of your points and am not trying to argue with you. I don't know why you are being so hostile in a learning environment. I never stated I was an expert and just came here for some information and guidance. You never had to post in this topic. It is very discouraging to read one of your posts putting me down. You have probably been rude enough to newbies like that they have quit tinkering with electronics altogether.

EDIT:

Just checked the voltage across a few of the buttons. They all jump around from 0.000V to 0.010V when not pressed and are obviously a steady 0V when pressed. This is using a UNI-T UT61E multimeter.

16

I don't know why you are being so hostile in a learning environment. I never stated I was an expert and just came here for some information and guidance. You never had to post in this topic. It is very discouraging to read one of your posts putting me down. You have probably been rude enough to newbies like that they have quit tinkering with electronics altogether.

Anyone who quit electronics because I offended them really shouldn't be in electronics.
To your credit you did say this (in Reply#4):

I have a rudimentary understanding of electronics as I went to tech school for instrumentation engineering but I only had a couple of beginner electronics classes.

FYI, you need to check your ego at the door when you post here on the forum. If you have a rudimentary knowledge of electronics , it stands to reason that most of what you are thinking is wrong or inadequate and someone will need to set you straight. To post on a forum of experts and then complain they were rude to you for admonishing you for not stating at the beginning that you have a rudimentary knowledge of electronics is counter productive. 90% of what we do is correct newbies. As I pointed out , it saves time if we know this because otherwise we give answers the poster doesn't understand. One of the points I made is that you have next to no knowledge of electronics and you come to a forum of experts and when they make a recommendation, you question it:

I just looked into FETS/Analog switches and it looks like they will add to much resistance when closed. Mechanical relays seem to be the way to go in this case.

If you know so much, why are you asking us ?
The reply to your above comment was this:

Actually, they will do just fine! What you illustrate are typical "chiclet" type switches with a carbon contact whose resistance is quite significant, so anything less than 100 Ohms is likely to do perfectly well.

We can do this the easy way or the hard way.
The easy way is you tell us up front that you're a beginner and we work with you on that basis.
The hard way is you don't tell us and we waste time and then we make suggestions and you question them and so on. This isn't a social media forum. This a technical forum. If you come here for technical advice, follow it when it is given, or solve your problem on your own, because it is a waste of our time to make recommendations if you are just going to dismiss offhand with nothing to support your reason for doing so. If you at least tried the recommendations (which, by the way, were not mine)
I would not be saying this. It is the fact that you dismissed them based on ignorance that prompts me to say it.

17

MrSHME:
Just checked the voltage across a few of the buttons. They all jump around from 0.000V to 0.010V when not pressed and are obviously a steady 0V when pressed. This is using a UNI-T UT61E multimeter.

To understand at what voltage they are operating, you need to check from each side of the button to ground.

That you measure negligible voltage across them indicates a couple of things; that they have pull-ups of a very high resistance and/ or that they are multiplexed so that the power is only applied very briefly to each one.

Either of these things implies that they are operating at no more than 5V and that a "contact" resistance of 100 Ohms or more would be quite satisfactory. To fully determine what is happening you would need to measure the voltages and waveforms using a CRO. (And if you do not know what a "CRO" is, raschemmel will have even more to say. :astonished: )

18

Oh boy, don't get me started. I got mine for $95 at HSC

Now I may be incorrect but I think the circuit has to be isolated as it reads resistance and I don't think I should introduce any extra current or voltage to it.

At the risk of being actually helpful, I would point out that the current is a function of the voltage and resistance, as expressed by Ohm' Law (V = I*R => I = V/R)

I would say that to be more accurate, the chip is reading the difference between the voltage drop when the button is not pressed (V= I*R) and the voltage drop when the button is pressed (V = 0, by because the contacts of the switch are at the same potential when pressed)

I would submit that if you , for reason we can't imagine, were to decide you wanted to actually KNOW what effect resistance has on your circuit board, you could easily do that by inserting a potentiometer (a low value such as 300 ohms, or at most 500 ohms) IN SERIES with the relay contacts you currently have and recording the resistance across the series pot (wired as a 2-pin rheostat) , starting at 0 ohms ,(measured with the power off) and increasing in resistance at 20 ohm increments, each time turning off the power to record the value, then turning on the power to test the board using the normal system test procedure. Collect the data, put it in an Excel file , save it as "
"Allowed file types: doc, gif, jpg, mpg, pdf, png, txt, zip, c, h, cpp, ino, pde
Restrictions: maximum total size 1 MB, maximum individual size 1 MB"

and post it as a table, that shows the increasing resistance values and on the same line the operational status of the circuit board when operated with that specific amount of resistance inserted;
ie:
R Status
0 OK
20 OK
40 OK
60 OK
80 OK
100 OK
120 FAILED

This test should yield similar results regardless of which switch you insert the series resistance in since the electrical properties of the chip are the same for all the switches.

By performing this experiment, you could INCLUDE/EXCLUDE relay alternatives that have been recommended in this post. It is only necessary to have one pot , since the data collected is going to be the same for any of the switches, it makes no difference which switch you choose to use for the test.

P.S.- Don't tell anyone I told you this. I wouldn't want to ruin my reputation of not being helpful...

19

@raschemmel,

FYI, you need to check your ego at the door when you post here on the forum. If you have a rudimentary knowledge of electronics , it stands to reason that most of what you are thinking is wrong or inadequate and someone will need to set you straight. To post on a forum of experts and then complain they were rude to you for admonishing you for not stating at the beginning that you have a rudimentary knowledge of electronics is counter productive. 90% of what we do is correct newbies.

Your posts do come across as being somewhat rude, and I could see folks being turned off by them. You could convey information in a friendlier tone (hard to get across in typing/text I realize). Answer simple questions with simple answers, provide helpful suggestions, act more like a teacher and less like an overlord. No one needs admonishing.

20

I'll try changing my water filter...