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Stupid Question : Reversed & Time Limited Relay?

JT_3k

Nov 5, 2014
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Hi. I've a bit of electronics experience, just enough to get me in trouble.
I'm planning a long-game type prank on one of my friends. Next year, when we're both at Le Mans 24h I'm going to fit some kit to his race car that plays a 5-second (inoffensive) MP3 every time his brake lights go out. I need to get him back after last year. I've already got and sourced a pair of particularly loud passive 8ohm PA horns, a small automotive audio amplifier that will run the horns, a wiring "take off" loom to gain my switched 12v/earth (and cause strictly no damage to the car - that's not the aim here) and a programmable greetings-card sound module (under it's own battery power) that plays when the contact switch is closed. I have the entire thing running perfectly on a bench in isolation.
I however need help figuring out the electronics to trigger the module as the line voltage in the brake lights drops to zero (not as it goes high - I'd easily be able to do that with just a standard relay). I can easily interrupt the bullet connector to the central high level brake light without damage but have no idea what goes between. Google seems to suggests a 555 but I'm unsure?

Any help REALLY appreciated.
 

Supercap2F

Mar 22, 2014
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Mar 22, 2014
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You can actually do it with a SPDT relay! :) Just hook up the normally closed side to your controller, so when the break turns on it will flip off your controller.

Also might not want to load down the the light bulb socket too much, so you could add a NPN transistor to switch the relay on. Just hook the transistors base trough a resistor to the light bulb, and it's collector to the relay, and it's emitter to GND. Hook the other side of the relay to the positive supply. Make sure you have a diode hooked up to it so that it's cathode it hooked to the positive side of the relay, and it's anode hooked to where the transistors collector meats the relay. For info on calculating the base resistor see here.
Dan
 

JT_3k

Nov 5, 2014
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Thanks. That makes sense. SPDT would work to instanciate a signal when the brake light switches off (I should have thought harder). As to the timing circuit, to only hold that circuit open for 5-15 seconds, I assume hooking that into a "555" is my sole option? I have a diagram that I think I can follow but I assume there's nothing more simple? Otherwise I'll just keep drawing power through that circuit and leave that sound piece open. However, I guess that's the least of my worries when the amp will be permanently on with the ignition anyway....
 

Supercap2F

Mar 22, 2014
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Ohh, I didn't realize you wanted a time delay. Yes you could do it with a 555, but why not just make the whole set up turn on when he pushes the break, instead of when he lets off?
Dan
 

JT_3k

Nov 5, 2014
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Sorry. Specifically the friend is known locally as "Captain Chaos" after the character in the Cannonball Run movies acted by Dom DeLuise. The complete plan is (in tandem) to install six large 8 ohm PA speakers under his car along with an eBay special car CD player and a cheap amplifier. Four of the PA horns will be hooked to the CD player which will loop the "Captain Chaos Strikes Again" theme music at volume whenever powered from underneath the car. Two of the PA speakers will be hooked to the amp which will be fed by my "greetings card MP3 circuit". I'll probably use some sort of eBay special RF remote controller to control the switched live to the setup, and install it all where his fuel tank used to be (pseudo racecar with a fuel cell in the boot).

The important bit of this is that the greetings card circuit will have a sole five second sound clip of the character shouting his charge call - "Dum dum DUMMMMMM!". If I can get the electronics to trigger when he releases the brakes (i.e. when he's setting off) the sound effect will be used in a similar way to the character in the film shouting it as he 'runs into battle'.

As the MP3 Card Circuit is 5v and the car is 12v, for the amount of time it's likely to spend on there, I'm not too worried about stepping the voltage around to power that also - I mean getting a clean 12v from the car's (likely)14.4v would be a pain I can do without.

My concern was that with these MP3 card circuits, they often don't take well to being left "on" and if you do it over 90 seconds, can sometimes take it as a cue to record, thus overwriting my sound effect and ruining the entire thing. Hence why it needs to be "trigger when off" (SPDT is perfect) but also ideally timed above 5s and below 120s.
 

Supercap2F

Mar 22, 2014
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So you want the card circuit to turn on when he lets off of the breaks, and stay on for about 5-120s (but no more then 120s). Have a look at the 555 in it's monostable mode. Just remember that the 555 is very inaccurate when it's a monostable.
Dan
 

JT_3k

Nov 5, 2014
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Thx. My limited Google searches and basic understanding suggested a 555 and further reading Monostable, but was hoping there might be something more simple as my electronics skills aren't great. TBH, inaccurate is fine, as long as I can confirm more than 5 sec but less than 120s. Much appreciated.
 
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