Electronic Relay Latching Circuit


I've spent a lot of time thinking about how I want to wire my conversion. The electrical system will consist of a combination of 12 volt DC, 24 volt DC, and 110 volt AC. It's important that the system provide the greatest functionality, but have a simple interface. It must be easy to install and relatively inexpensive to build. The system can't be overly complicated and must be easy to maintain.

As I've thought about the many different circuits that I will need, one question has always come to mind. How can I control a single electrical circuit with multiple switches scattered throughout the coach? In other words, how can I turn on a light or pump from one location, and turn it off from a different location?

If I were designing a 110 volt system, the answer would obviously be to use 3-way and 4-way wall switches. These are readily available and fairly simple to integrate into the wiring scheme. Unfortunataly, these switches are designed for AC voltage, and should not be used for DC voltage. Another problem with using 3-way and 4-way switches is that once installed, the circuit does not lend itself to changes. You can't change the function of individual switches without rewiring the entire circuit(s).

One method of getting around the above problems is to use relays to control individual circuits. Low voltage switches are used to activate the relay coils, which in turn activate the circuit being controlled. In this scenario, each circuit is wired to a central location where the relay is installed. Switches are installed in convenient locations within the coach, and are connected to the relays via a single wire, or pair of wires. The switch is used to activate the relay coil, which closes the relay contacts and activates the circuit. Only a small current is required to activate the coil, so smaller wires can be used in the installation. Each relay can switch several amps of current to the circuit load. Pretty simple... until you try to use 2 distinct switches to control a single relay and load circuit. How do you activate the circuit with one switch, and deactivate it with a separate switch?

The answer lies in momentary switches and latching relays. In a momentary switch, the actuator is spring loaded so that the switch contacts do not stay closed when the switch is released. The swtich contacts return to the normally open (off) position automatically. Momentary switches maintain the connection only for the moment that you hold it closed. However, using momentary switches with standard relays will not solve the problem above, since releasing the switch causes the switch contacts to open, which deenergizes the relay coil, and deactivates the circuit being controlled. So in addition to the momentary switch, you also need to use a latching relay.

Latching relays are a type of relay that change state when pulsed with a current to the coil, and remain in that state, even if the activating current to the relay coil is removed. When the coil is pulsed again, the relay changes state again.

So this sounds like a winner. You pulse it with a switch. The relay activates, closes the contacts, and turns on the circuit. When you release the switch it returns to it's normally open (Off) position, but the relay remains activated and it's contacts remain closed, keeping the circuit on. When you want to turn off the circuit, you pulse the relay coil again, which causes the relay to change states and opens the contacts, thereby turning the circuit off. Perfect! So where do I find momentary switches and latching relays?

Momentary switches are not difficult to find, and are not terribly expensive. Latching relays, on the other hand, are very difficult to find, and are very expensive. This doesn't fit into my general criteria of being inexpensive yet easy to build and maintain.

Then one day while surfing my favorite bus related sites on this wonderful media known as the World Wide Wait, I stumbled across an article on Coach Conversion Central, titled Remote Light Control by Dave Martin. In it, he explains how to build a 12 volt relay circuit that can be used to remotely control lighting or a water pump. His circuit involves feeding the relay load output through a transistor and back to the relay coil to maintain current on the relay coil when the switch is released. This keeps the coil activated, which in turn keeps the relay contacts closed. It's simple, straightforward, and exactly what I've been looking for. The only problem is I don't know much about transistors. No problem! I have a friend who is a fine electrical engineer. I'm sure he'd be happy to quickly explain them to me! He thinks I'm nuts anyway, but he gets a kick out of my ideas, and he enjoys seeing a software engineer playing with circuit design!

So, after some tutorial help on understanding how the circuit works, and an email or two to Mr. Martin asking questions about his design, I set about modifying it to work on 24 volts. I ordered some parts from Jameco Electronics and built a prototype on a small breadboard. This was connected to a relay, and an prototype wafer was attached to it for a simple load. I was delighted when it worked. I then added a couple more circuits to the breadboard, and attached two relays and two 12 volt fans in series with two momentary switches. I wired everything up with the relays and latching circuits to produce a 2-speed fan control. It worked great! (I envision eventually using the fans and 2-speed fan control on my diesel fired heating system.)

So, how do I package this circuit? I had found that Jameco Electronics had empty 8-pin relay enclosures available at a reasonable price. This seemed like a viable solution. The relays in the bus are 8-pin models, and and I thought the enclosures might make nice packaging for the latching circuit. Bases are readily available, large enough to work with, and would make it easy to mount the circuit. The plastic case would protect the circuit from the elements, and would make it simple to integrate into the coach electrical circuits. After some playing around with the design and various ideas about how to mount a circuit card into the relay enclosure, I was able to fit two of the latching circuits onto a small PC card wafer which I cut to fit into the relay enclosure. Note the two red LEDs hanging off the edge of the wafer. These were added to provide a visual indicator that the circuit is activated. There will also be LED indicators at each switch location.

With the layout confirmed, I set about soldering up a prototype circuit card. I chose to install this wafer into a salvaged relay enclosure, rather than use a new enclosure. I have several 1.4 volt relays that I bought under the impression that they were 14 volts. (By the way, if anybody needs 1.4 volt relays, I have several available and can make you a great deal on them.) The 1.4 volt relays cost me less than buying a new enclosure from Jameco Electronics. The only downside to using these used enclosures is that they have writing printed on them, and a hole in the enclosure where the relay mechanism was screwed to the enclosure. I figure I can find something to plug the hole with if it becomes necessary. Since they will be installed in the bays, and possibly in an enclosed junction box, it probably won't even be necessary to plug it.

The prototype circuit was connected into my test setup and worked very well. Notice the breadboard still connected into the circuit. This was used to provide an LED indicator on each switch, to simulate how the circuits will operate in the coach.

Using the prototype circuit, I was able to take some current measurements and refine the design. A couple diodes were added to the design for relay coil back EMF dissapation. This aids in a clean installation as the diodes no longer need to hang off the relay bases. I also added a diode to the ground input after accidentally reversing the polarity of the connections to the battery and frying the transistors.

The first production unit was then built.

Components were soldered on using point-to-point soldering on the back of the wafer. Solder bridges were used to connect components and jumper wires were soldered into the circuit as necessary.

The production unit was then mounted into a new relay case and connected to my test setup. I attached a couple prototype fixtures to the relays for loads. One switch was connected to one of the circuits, and two switches were connected to the other circuit. The switches are CarlingSwitch Contura rocker switches in a Mom-On / Off / Mom-On configuration. The circuits worked great. Pressing the switch in one direction activated the circuit, turning on the light fixture, which stayed on. Pressing the switch in the opposite direction deactivated the circuit and the light went out. On the circuit which had two switches attached, I could turn on the lights with either of the switches, and turn it off with the other. This is perfect!

This electronic latching mechanism is exactly what I was looking for. It's inexpensive and very easy to build, and should perform well in my electrical scheme. It's designed for 24 volt loads, and switches. Since I will have some 12 volt needs in the coach (lighting, pumps, etc), I will either modify this circuit to accommodate both 12 volt and 24 volt circuits, or will build a 12 volt model. I'm also contemplating using the design to control 110 volt AC loads, also. I think I can use the 24 volt latching relay circuit to control a 24 volt DPDT relay with 110 volt AC contacts by routing 24 volts through one side of the contacts back to the latching circuit, and 110 volts through the other side of the contacts to the load.

I've also begun work on a schematic to be used to test the relays and latching circuits. It will be located near my relay installation, and will help troubleshoot any problems I have with the relay system. With this test fixture, I should be able to plug in a relay, push a button, and immediately know if the relay or latching circuit is working properly. When I get to that point, I'll do a writeup on it, also.


020304 - Well, I finally got the latching relay circuit completed. I have made a few modifications to it, including the addition of a couple more diodes on the load line to prevent the load from being activated by the switch. This will prevent overloading of the switch control wire, which can now be very light duty wire. I'm considering using 22 - 24 awg multiconductor cable to route my switch signals from the control panels to the relay panels.

I changed one of the indicator LEDs from red to green. Since there are two circuits on one wafer, this allows me to know immediately which circuit is activated.

My original prototype and production units were modified to the latest design, and a few more production units were built this winter. It takes me about an hour to solder up a single unit, and they are actually starting to look like I know what I'm doing (thereby proving that looks can be deceiving). Here's some photos of the latest production units.

I also have drawn the schematic in CAD and posted it here for anyone that might wish to build a similar relay system, or improve upon the design. This schematic may be distributed freely and used for non-commercial purposes only and may not be sold, published, or used for any commercial purpose without the express written consent of the designer (and that would be me!). If you have any questions about the design, .