# latching with a timed relay

I am an ultra newbie, so please excuse my design/drawing here and I am sure my terms are not correct, but I want to learn. Here is what I am trying to do and a diagram. I think I got it right, but would love some advice.

Object: To turn on a light and have it turn off automatically after a period of time by a time delay relay.

Ground: Power Source Negative (green) to negative on relay. Lights Negative to negative on relay.

Power: Power Source Positive (red) to positive on relay through a button. Jumper from positive on relay to common on relay.

Button: Momentary Push Button making the initial connection.

Lights: Positive on lights (red) connected to first terminal on time delay relay. After an amount of time, this terminal is switched off and it defaults to turning on terminal 2 which will not be used.

Latching: Both connectors on momentary push button (blue) latched to terminal 1. This is to create a by-pass creating a latched connection to allow the lights to remain on even though the momentary push button is released breaking the initial connection.

Relay Reset: Once terminal 1 is turned off, it will sever the latched connection, thus removing all power from the relay resetting it.

My Biggest Questions is, "is the latching going to work when I release the momentary push button and will it shut off power to the relay once connection 1 is terminated once the time has expired??

• A couple of questions. How long are the lights supposed to stay on, and what turns them off? – Willis Blackburn Mar 12 '16 at 4:18
• What are the lamp voltages and currents/wattages and are they AC or DC? – EM Fields Mar 12 '16 at 4:54
• I am working with a 12v DC battery. The lights are 12v auto lights. I will be using this relay: oznium.com/12v-delay-timer-relay the one without the digital display. I will have it activate for 3 - 5 seconds. – user3217101 Mar 12 '16 at 5:03
• Here are the lights I have: amazon.com/Bright-Warning-Emergency-Beacon-Hazard/dp/B00WFZ9WB6/… – user3217101 Mar 12 '16 at 5:06
• Basically, it is a project for warning strobe lights on the front of a scooter for saftey when someone is backing up from a driveway or approaching an intersection. We can't have modulating lights in CA, so I was wanting to install a pushbutton on my front panel I could just press and have the lights strobe for about 3 seconds and then shut off, not needing me to flip the switch again to turn them off or reset the relay. – user3217101 Mar 12 '16 at 5:08

## 2 Answers

I don't know if this helps, but if you are looking to turn on the lights for a few seconds to a few minutes, you could use the button to trigger a 555 timer circuit that drives the relay. This one will turn on the lights for about a second. Increase R4 or C2 to increase the delay; there's a chart in the data sheet that you can use to figure out the right values to use. R2 will depend on your relay.

simulate this circuit – Schematic created using CircuitLab

• Yes, I am wanting them to stay on just a few seconds then turn off. I am not at the level of designing my own circuit board yet, so I want to use off the shelf stuff like: oznium.com/12v-delay-timer-relay with these lights: amazon.com/Bright-Warning-Emergency-Beacon-Hazard/dp/B00WFZ9WB6/… – user3217101 Mar 12 '16 at 5:10
• The red 0-10s timed relay is a packaged version of the circuit I've shown here. The 8-pin IC is the 555. The big cap is C2 from my diagram, and the screw-turn potentiometer is an adjustable version of R4. So you don't need to build the thing at all. Can't you just attach your switch to the terminals and party on? – Willis Blackburn Mar 12 '16 at 5:23
• Im hoping. I guess my big question is the latching. Im hoping tje way i jave this wiring would allow power to stay on after the button is released? – user3217101 Mar 12 '16 at 5:36
• Okay, I see your problem. You don't have the button labeled SW1 in my diagram. The VCC and GND inputs power the board. When you turn it on, you get one timer cycle. Okay. Try this. Hook up the lights to the relay end of the board (COM and either CB or CK, not sure which) and then attach VCC and GND to the power supply. The lights should come on for a few seconds. If not, change CB to CK or vice-versa. – Willis Blackburn Mar 12 '16 at 5:51
• Once you get that to work, take a piece of wire and connect pin 2 on the IC to GND. If you hold the chip so the writing on the top is right side up, pin 2 is on the bottom, second from the left. Pin 2 is the trigger pin. Connecting it to ground should cause the 555 to restart the timing cycle. Did it work? If not, try connecting pin 4 (bottom, fourth from left), the reset pin, to GND. If either one of those worked, fix your button so it shorts that pin to ground and you're good to go. – Willis Blackburn Mar 12 '16 at 5:53

This is @Willis Blackburn's schematic, but edited so the circuit will work.

R1 was added to provide DC restoration for C1, which was added in order to make sure that no matter how long SW1 was made, U1 would time out.

D1 was added to clamp the spike generated by SW1 opening tom V1+, (3 to 18 volts for a 555)

D2 was added to clamp the negative spike generated by RLY1's coil when U1's output goes low, therefore keeping the magic smoke from getting out, and

$$\tau = R_T\Omega \text{ } C_T F$$

is the timing for a 555 one-shot, where $\tau$ is the timeout in seconds, Rt is he timing resistor in ohms, and Ct is the timing capacitor in Farads.

Megohms and microfarads also works...

• I have learned more from this thread tonight that I have days on my own. Thank you all! – user3217101 Mar 12 '16 at 7:38
• Working is overrated. :-) I did build the circuit before posting it, but I used an LED instead of a relay. Thanks for the fixes. – Willis Blackburn Mar 12 '16 at 14:12
• @WillisBlackburn: An upvote is a nice "thank you" :-) – EM Fields Mar 18 '16 at 0:36