Radio Button Relays

[frankwas]

Hi All!!

How is everyone? I have a circuit that I was thinking about building, but I can't figure out how to do it. I would like to build a circuit using about 5 relays and 5 push buttons. Then when push button no1 is pressed, relay one comes on, when push button no2 is pressed relay 2 goes on, but 1 goes off. The concept is that works like those old style radios that you could only have one button depressed at a time. Also like radio buttons in forms.

Does anyone have an idea how I could do this without using a PIC?

Thanks in advance!

Francois.

 

[Bob Scott]

Yes. You wire the momentary switches so that each one turns on its own relay. Wire all of the relays so that when activated, they latch themselves ON. Supply them all with the same constant current power supply. Limit the current so that it will supply only enough current to keep one relay on and latched.

When you manually press button #2 so that relay #2 turns on, the supply does not have enough current to keep any other relays on at the same time, so all other relays turn off.

I used this method a long time ago for car stereo display switchers.

 

[frankwas]

Thanks Bob!!

That makes so much sense! I'm gonna try it immediately! I really appreciate the reply!

 

[eblc1388]

When you manually press button #2 so that relay #2 turns on, the supply does not have enough current to keep any other relays on at the same time, so all other relays turn off.

Sorry to be a bit skeptical about this idea.

Let's suppose the first relay coil is fed via a constant current source and latched, its coil has the full coil rated voltage.

Parallelling a second relay to the supply of this relay would result in the coil voltage of both relays drops to half the previous value, as the same current is now shared between relays.

For most relays, the latched one would still remain latched and not dropped out at half its rated coil voltage and the latter one won't even pull in because it has only 50% of its coil operating voltage.

 

[KMoffett]

Bob,

I was rooting for you, and I relay wanted the relay idea to work. So, with LChang's caveat, I went down to the workbench. 24v relays held on down to <8v.

My guess is, with 5PDT relays and a lot of wire you could do it...but I'm not going to.

I was playing on another thread with a circuit to do a 1-of-3 pushbotton. The attached is one of my results. This is expandable to 5 or more pushbottons by adding pairs of gates and diodes...and replacing the resistors and LEDs with mosfets and frankwas's relays. The PICAXE verson was much cleaner...but he said no PIC's.

Ken

 

[crutschow]

Sorry to be a bit skeptical about this idea.

Let's suppose the first relay coil is fed via a constant current source and latched, its coil has the full coil rated voltage.

Parallelling a second relay to the supply of this relay would result in the coil voltage of both relays drops to half the previous value, as the same current is now shared between relays.

For most relays, the latched one would still remain latched and not dropped out at half its rated coil voltage and the latter one won't even pull in because it has only 50% of its coil operating voltage.

A possible way around that is to place a resistor in the latch circuit from the relay contact to the coil. The push button circuit would have no resistor and would thus apply full voltage to the relay being latched.

By proper selection of the resistor value the relay not selected will drop out when any other relay is selected. For example the 24V relays held to <8V so select a resistor that applies about 10V when latched. This should drop the voltage well below 8V when another relay is energized.

 

[KMoffett]

I said I wasn't going to do it...but laying in bed at 3AM, my brain and I don't always agree.

Five pushbuttons and five 4PDT relays. Expand or decrease...it's "N" relays with "N-1"PDT contacts.

The contacts and coils are not physically oriented in the schematic. The contacts above each coil are on the 4 other relays. Just an easy way to visualize what's happening.

In the schematic, coil on relay C is powered through NC contacts AC, BC, DC, and EC on the other four relays. The contacts on relay C...CA, CB, CD, and CE...are in the NO position ...opening the path to the 12v on each of the other coils. If the button for relay A is pushed all its contacts...AB, AC, AD, and AE...go to NO removing power from the other coils. The NC contacts on relays B, C, D, and E...BA, CA, DA, and EA...provide power to A's coil, latching it on. At power-up all the relays would be off, until a button was pushed.

The output is just whatever you put parallel to the relay coils.

This is why we now use PIC's!

Ken

Another 3am realization. When first powered up, all the relay contacts are in the NC position, so all coils see 12V at the same instant. All coils try to pull their contacts to the NO position. The first relay to get it's contacts moved would be latched. This could be random, weighted, or chatter. I think if an electrolytic cap were placed across one of the pushbottons, it would hold that one on long enough to assure it was always selected. This is still playing with the original request of 5 relays and 5 pushbottons.

 

[eblc1388]

This is why we now use PIC's!

Or a simple 4017.

Pressing any button (upto to 10 individual buttons) will set that 4017 output high and turn ON the associate transistor.

Any clock source with frequency higher than 1KHz can be use.

 

[Bob Scott]

Sorry to be a bit skeptical about this idea.

Let's suppose the first relay coil is fed via a constant current source and latched, its coil has the full coil rated voltage.

You can lead a horse to water but you can't make him drink. If he calls you a liar, he can find his own water.