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SILICON CHIP Mini Projects #005 – by Tim Blythman Self Toggling Relay Here’s a simple circuit, using just one relay and a handful of passive parts, that allows you to toggle or switch the relay off and on with just one pushbutton. It demonstrates some of the finer details of working with relays and can be used to control a wide range of devices. O ver the years, we have had several requests for simple circuits that allow a relay to be switched on and off by pushing a button. Some readers sent in suitable circuits, but they all involved multiple relays. This design achieves that goal using just one DPDT (double-pole, double-­ throw) relay, a common type. It provides a free set of ‘dry’ contacts, meaning they are not connected to any circuitry or a power source. You are free to do whatever you want with them, within the limits imposed by the relay’s ratings. It might be possible to build this circuit using a single-pole (SPST) relay if the thing you wanted to switch was completely independent of the relay’s power source. Still, DPDT relays are inexpensive, and using a free set of contacts is safer. The relay Our design relies on a property of relays that we can demonstrate with the circuit in Fig.1. A capacitor is connected across the relay’s coil and charged via a resistor fed from the normally closed (NC) contact. When power is applied, the capacitor charges until the relay’s armature pulls in. The contacts open, the capacitor discharges until the armature drops out, and the cycle continues. It forms a ‘relaxation oscillator’. Scope 1 shows the resulting waveform for the Jaycar SY4065 relay that we are using. The blue trace is the voltage across the coil, while the red trace shows Scope 1: the blue trace shows the voltage across the coil, which rises and falls between the must-release and must-operate voltages. The red trace shows the relay state changing as the voltage does (with quite a bit of contact bounce at the transitions, as is expected). 46 Silicon Chip Australia's electronics magazine the relay state changing. Although it is a 12V relay, the contacts open at around 1V and close at around 9V. The relay data sheet lists those as the ‘must release’ and ‘must operate’ voltages. As you can see from the scope grab, this circuit toggles at around 10 times per second, so this relay’s minimum 100,000-cycle operating life would be reached in about three hours! If you want to apply this design to another relay, we recommend checking its data sheet first. Our circuit Fig.2 shows the circuit for the Self Toggling Relay. RLY1’s coil has a resistance of around 160W, and when power is applied, it has around 6V across it. That is less than the must-­ operate voltage, so the relay remains off. The capacitor charges up via the relay’s NC and COM contacts, reaching close to 12V after a few seconds. Pressing S1 places the capacitor directly across the relay coil. Since it now has 12V across its coil, the relay pulls in and the contacts change over. When S1 is released, the coil voltage returns to around 6V, above the must-release voltage. The capacitor now discharges to 0V via the NO and COM contacts. This takes around a second, since it will have discharged slightly while S1 was pressed. If S1 is pressed again, the siliconchip.com.au Fig.1: this is the circuit we used to test our relay before building the prototype. Scope 1 shows the resulting waveform. We suggest you don’t build this as it will cause the relay to toggle rapidly, possibly wearing it out quickly. reverse happens and the relay drops out, returning to the earlier state. Effectively, we are using the hysteresis of the relay coil voltage (the difference between the must operate and must release voltages) to maintain its state and using the capacitor to change the state. The capacitor charging time sets the maximum toggling rate, about once per second for the chosen components. Since the relay is not operating at its full rated voltage, the contacts are not pulled in as tightly as they would otherwise be. This means the relay may be more susceptible to vibration and shocks and might drop out (or in!) if subjected to rough conditions. Also note that if power is removed, the relay will return to the released state almost immediately; this is a nice safety feature. Construction We have used a socketed relay to minimise the amount of soldering needed. The physical arrangement has been kept similar to the Fig.2 circuit diagram to make it easier to follow. You could use a smaller relay laying on its back and solder the other components to its leads, ‘dead bug style’. The circuit is also easy to assemble on a breadboard or prototyping board. Just be aware that other relays might have different pinouts or component requirements; we’ll discuss that later. siliconchip.com.au Fig.2: the Self-Toggling Relay circuit diagram, laid out similarly to our prototype. The component values have been chosen to work with the selected relay; different relays will likely require different values. The Parts List reflects what we have built, but several alternatives exist. See the photos for how the prototype was wired up. We used black wire for the two terminals connected to the negative end of the 12V supply, while the red wire connects to the positive of the 12V supply. Wire up the lower set of contacts first, as they will be hard to get to once the upper components (particularly the capacitor) are fitted. We’ve mostly made the other connections using the component leads, with some extra wire in some places. The blue and white wires connect to the switch terminals. Testing Hook up the 12V supply; nothing should happen right away. If the relay starts chattering, disconnect the power supply and check your wiring. You might have inadvertently made a circuit more like Fig.1 than Fig.2. A wrong value for the 1kW resistor might also cause chattering. Wait a few seconds, then press the pushbutton and confirm that the relay toggles. Wait another second and confirm that it toggles back when the pushbutton is pressed a second time. In that case, the circuit is working. You can use the second set of contacts as though they are an SPDT switch, or you can use either half (NO & COM or NC & COM) like an SPST switch, depending on whether you want it to default to open or closed when power is not applied. If you want to add an indicator light, a 12V globe or 12V LED could be connected between the COM and NC terminals. This will light up when the relay is pulled in. Similarly, a globe or LED connected between the COM and NO terminals will light up when the relay is off. Alternatives If you need another button that will always switch the circuit on or off, you could add another pushbutton, 1000μF capacitor and 1kW resistor and wire them up in almost the same fashion. In this case, instead of feeding the 1kW resistor from the relay’s COM contact, feed it from 12V for an ON switch or 0V (ground) for an OFF switch. That gives a circuit that can generate the necessary impulse, but it will always have the same effect instead of toggling. Keep in mind that pressing more than one switch simultaneously might connect capacitors charged to different voltages, possibly running high currents through the switch contacts and damaging them. Other relays The 150W resistor value was chosen to set the coil voltage between its must-­release and must-operate voltages. For other relays, a resistor with a similar resistance to the relay coil is a good starting point. By the voltage divider equation, this will put about Parts List – Self Toggling Relay (JMP005) 1 DPDT 12V relay (RLY1) [Jaycar SY4065] 1 relay socket base to suit RLY1 [Jaycar SY4064] 1 momentary SPST pushbutton switch (S1) [Jaycar SP0710] 1 1000μF 25V electrolytic capacitor [Jaycar RE6230] 1 1kW 1/2W resistor [Jaycar RR0572] 1 150W 1W resistor [Jaycar RR2554] 1 12V DC power supply various short pieces of stiff wire to make connections Australia's electronics magazine June 2024  47 This simple circuit is a proof of concept. Still, we think readers will find it handy when they need to toggle a relay using only a single momentary pushbutton. half the supply voltage on the coil. If the data sheet does not mention a figure, measure the coil resistance with a multimeter. Note that we used a 1W resistor here; you should check the power dissipation if making circuit changes. Your circuit should run from the voltage the relay is rated for. Don’t try to power a 5V relay from a 12V supply! An early prototype we built used a smaller relay and we found that a 100μF capacitor could provide enough impulse to toggle the relay. The exact value depends on the relay, so we advise experimentation to find a capacitor value that works consistently. The second resistor (1kW in our case) must have a high enough value to avoid substantially changing the coil voltage while the pushbutton is pressed. A value at least five times higher than the first resistor should work well. The combination of the 1kW resistor and 1000μF capacitor (as used in our circuit) dictates the maximum rate at which the pushbutton can toggle the relay. Larger values will mean a longer SC wait time between presses. 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