1/4 second 12v lamp delay?

[swedluap]

Forgive me for being a complete noob.

I want to build a circuit to delay an automotive light bulb by merely 1/4 to 1/2 second. I want to keep it small and simple. I have no idea where to start.

 

[Sceadwian]

Have the switch connect a simple low pass filter (a big resistor in line with a modest capacitor connected to ground) to a Schmitt triggered buffer. When the switch is triggered the capacitor will take time to charge up to the Schmitt buffer's trigger voltage. If you use an SPST switch you'll also get the delay during turn off. If you want the on delay with instant off you'd have to use a DPDT switch so that when the switch was off the Schmitt buffer input was shorted directly to ground after the resistor. The Schmitt buffer itself would drive a relay or transistor to actually turn the light on. If this description isn't enough let me know and I'll draw out a schematic for you. All of these terms are searchable on Wikipedia if you're really confused.

 

[k7elp60]

Forgive me for being a complete noob.

I want to build a circuit to delay an automotive light bulb by merely 1/4 to 1/2 second. I want to keep it small and simple. I have no idea where to start.

Which automobile bulb are you talking about? Is it a headlamp, taillight,or panel lamp?

 

[swedluap]

Have the switch connect a simple low pass filter (a big resistor in line with a modest capacitor connected to ground) to a Schmitt triggered buffer. When the switch is triggered the capacitor will take time to charge up to the Schmitt buffer's trigger voltage. If you use an SPST switch you'll also get the delay during turn off. If you want the on delay with instant off you'd have to use a DPDT switch so that when the switch was off the Schmitt buffer input was shorted directly to ground after the resistor. The Schmitt buffer itself would drive a relay or transistor to actually turn the light on. If this description isn't enough let me know and I'll draw out a schematic for you. All of these terms are searchable on Wikipedia if you're really confused.

A schematic would be great. If you could "dumb it down" that would be better

Which automobile bulb are you talking about? Is it a headlamp, taillight,or panel lamp?

It is a 3157, Marker/Turn bulb.

 

[swedluap]

Although, the switch is already taken care of by a flasher. My truck has two bulbs on each side that act as turn signal bulbs, I just want to delay one of them by 1/4 to 1/2 second (variable POT would be a nice touch, too) to give it the "sequential" effect, and I don't have a lot of room to work with, so a big circuit is what I want to avoid.

 

[k7elp60]

Here is a delay circuit that works. With a fixed resistor of 1M instead of the potentiometer and the 10k resistor the time is about 1/2 second. The polarity is important, but the switch is not necessary. About 1/2 second after the power is applied the lamp turns on.

 

[swedluap]

Wow, thanks!! I think I actually understand that!! What size pot would I use in place of the 1M 10K to get 1/4 to 1/2 sec variable?

 

[k7elp60]

Wow, thanks!! I think I actually understand that!! What size pot would I use in place of the 1M 10K to get 1/4 to 1/2 sec variable?

With the values shown, that is the 10k fixed and the 1M you can adjust the time from no noticable delay to about 1/2 second. The actual time will depend on the actual value of the capacitor and the applied voltage. In reality how ever long it takes the capacitor to charge to about 5 volts to turn on the transistor.
With just a fixed 1M resistor the time is close to 1/2 second.

 

[swedluap]

Ok, I'm just about to build it. One more question. It looks like most of the load is handled through the transistor, so would 1/4 watt components be sufficient for the cap, 10k and 1m, or should I beef that up? If so, 1/2 watt? 1 watt?

 

[Roff]

Ok, I'm just about to build it. One more question. It looks like most of the load is handled through the transistor, so would 1/4 watt components be sufficient for the cap, 10k and 1m, or should I beef that up? If so, 1/2 watt? 1 watt?

I think you need a few more parts. As drawn, it might work well for a single closure, but for a repetitive circuit like a turn signal, the cap never has a chance to discharge fully if you make it large enough to give you an appreciable time delay. The diode solves this problem.
The NPN (any small-signal general purpose NPN) gives positive feedback, causing the MOSFET to switch rapidly and much more efficiently.
1/4 watt resistors are fine.

 

[swedluap]

Got it built and it works exactly as I had imagined. You guys rock!! I used Roff's design because I wanted to make sure the second bulb went off with the first. Brilliant!!

If you want to see the prototype in action, here it is.

**broken link removed**

 

[swedluap]

Update:

It works well with the engine off (12v) but the bulbs flash simultaneously with the engine running (14v). A voltage regulator should fix that, right? Or is there something else I should think about?

 

[Roff]

Update:

It works well with the engine off (12v) but the bulbs flash simultaneously with the engine running (14v). A voltage regulator should fix that, right? Or is there something else I should think about?

The slightly higher voltage should be no problem. The problem is probably noise on the supply when the engine is running. Cars are notorious for this. Unfortunately, you can't add a filter or a regulator without making your lamp dimmer. I tried to simulate the effect, but could not duplicate it. I suspect that it might not happen if you get rid of the NPN positive feedback. Try it and let us know if it works.
I suppose you could filter the noise with an LC filter, but it would require one or more high-current inductors.
BTW, in my simulations, I added 10k from the source of the MOSFET (switched +12V) to GND, so that the cap still has a discharge path after the MOSFET turns off. I have edited the schematic to show this.

 

[swedluap]

Sorry it took so long to give you results, but I have been out of town for the past week.

Removing the 2n3904 and the 100k did the trick. This results in the bulbs flashing in sequence with the engine off as well as with it on. With the engine running, the bulb delay is slightly shorter, but it still gives the desired effect. I have adjusted it to delay perfectly with the engine on and it looks nice (even with the longer delay) with the engine off.

Thanks for all the help!! I could not have done it without y'all!! Best of all, I had fun with this project, it always feels better when you make something yourself as opposed to buying it pre-made.

One more question. I want to make a pair of these for a buddies truck, I cleaned out the local Fry's of the IRF9530's (2), would the IRF510 be a good replacement? or should I order more 9530's?

 

[Roff]

IRF510 is NMOS. IRF9530 is PMOS. You need to stick to PMOS, but I would buy one with lower Rds(on). The 9530 is about 0.3 ohms. Lower on resistance will let it run cooler (although this may not be a problem, since the duty cycle is typically low). It will also let your bulb run closer to full brightness (also may not be an issue, though).

 

[swedluap]

Ok, there is no rush, so I'll just order more 9530's. Heat is not an issue at all, I let them run for 5 min to test that, and they aren't even warm.

You have done me well, my friend!! Thanks much!!

 

[Hero999]

Wow, thanks!! I think I actually understand that!! What size pot would I use in place of the 1M 10K to get 1/4 to 1/2 sec variable?

The MOSFET only conducts when the gate is below the source by about 5V.

The capacitor takes time to charge (this is determined by th RC time constant), when the voltage reaches about 5V the MOSFET turns on causing the bulb to light.

 

[jesfre]

I think you need a few more parts. As drawn, it might work well for a single closure, but for a repetitive circuit like a turn signal, the cap never has a chance to discharge fully if you make it large enough to give you an appreciable time delay. The diode solves this problem.
The NPN (any small-signal general purpose NPN) gives positive feedback, causing the MOSFET to switch rapidly and much more efficiently.
1/4 watt resistors are fine.

I am reading the diagram and I see it says 2.2 uF and I am not sure what to use because everything Im finding has different voltages. is there a particular voltage?

swedluap is there any way you can post a list of the exact parts you used and if possible links to everything but the resistors? this would be awesome. thank you guys.

P.S. sorry for being 6 years late.

 

[ericw2k]

I am also thinking about making the sequential tail light for my 11 Camaro, I came across this online 3-Step Sequential Module Box, will this work?

 

[Reloadron]

You would have done better to start a new thread. It will work as long as it is compatible with your existing light system, I would call the manufacturer and ask them if it is suitable for what you have.

Ron