89 Iroc Z

I am trying to install a small circuit that will make the dome like dim / fade out when you close the door. I have seen this feature in a few new cars. "86-red_dawg" refereed me to a site in my previous post that had the circuit diagram (below). Will this work on 3rdgen's because they have a negative dome light switch and it seems like this circuit is setup for a positive dome light switch.



http://www.electronic-circuits-diagr...carsckt3.shtml

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1989 Iroc-Z 5.7 350, Blue, T-Tops

Lightly Moded:
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-- Blue LED Bakelite Dash & Climate Control
- I am just some 17 Year old spending every dollar I make on my car.



[This message has been edited by 89 Iroc Z (edited December 09, 2001).]

RSpeed

You can use a relay to invert the neg signal to positive at the dome light circuit and the new "fading" circuit junction.You may also need a diode to isolate the two circuits.

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89 Iroc Z

Can anyone go into a little more detail please?

junkyarddog

who cares how the lights wired! it's not like you have to leave the wires as they are,leave the light there,switch the wires around.....or whatever and follow those instructions to a "t" ,use the same value resistors,match the IC part #ect... you can most likely fit all that on a 2x2" pre drilled & etched circuit board from radio shack or someplace like it. don't use radio shacks soldering irons though....complete junk!! I bought a 15watt pencil style iron from R.S. and it wouldn't even melt the solder!! I ended up with a 100watt Weller soldering gun which is great for automotive work,but I'd stick with something smaller like a 15watt for light circuitry work,espescially IC chip!! weller is about the only brand I'll mess with.

89 Iroc Z

I care how the light is wired so I can use the stock wiring. The circuit is setup for positive switch dome light. I am asking how I would use this circuit on a negative switched dome light.

SgtSkip

Do you have a schematic og the circuit? It all depends on what they are using to control the current to the light. You MAY run in to biasing problems with the reverse polarity but I doubt it since you probably are not reverse biasing the control portion of the circuit.

Dr. Pepper

I wouldn't attack it from the perspective of trying to invert what signal you have coming across the dome light since it will be removed from that part of the circuit. You simply need to provide a switched signal where you now have a ground/12V signal. You need a normally open relay. Connect the relay coil in place of your current dome light. The relay will be off (open circuit) when the dome light is off. This causes the op-amp to output 0V which does not turn on the 2N3055 transistor which in turn does not turn on the dome light. Should the dome light be activated by the car's factory wiring 12V and ground are present across the relay's coils turning it on. This closes the contacts of the relay sending 12V to terminal 3 of the op-amp which also causes 12V to appear at terminal 6 turning on the transistor and the dome light. When the dome light would have been turned back off by the car's wiring the relay now gets turned off opening the contacts again. This causes a controlled discharge of the 22uF capacitor which slowly turns off the transistor and thus the dome light. This is exactly what you are looking for. Just need a cheap NO (normally open) 12V relay from Radio Shack. Looking in their catalog part numbers 275-233 and 275-241 should both serve you well. Then just use a high amperage 12V source and wire in your dome light fixture to your new circuit. Be sure to heat sink the 2N3055 transistor well and to use heavy gauge wire comparable to the factory dome wiring. I haven't measured the current going to a dome light but I get the impression it is a fair amount.

SgtSkip

Oops, your schematic didn't load the first time I looked for it. Doc, don't you mean replace the switch in the circuit with a relay? Good idea keeping things simple.

89 Iroc Z

Could anyone tell me how many watt variable resistor am I should use for VR1 VR2 on the circuit diagram?

junkyarddog

the op-amp circuit will fade the light when it's turned off by the door switch,I'm assuming that you don't want that effect when your using the factory switch. simply leave the circuit as it is,and place the factory switch between the neg. side of the lamp and ground. this will only operate the lamp and not the fade. ckt. that way the lamp will be off when you switch it off,...no fading out. I'm assuming that the lamp is not grounded at the chassis,two wires +/- coming through the window frame down under the dash,right? there should be litlle rewiring neccesary.
use the highest wattage resistor available,5watts should do. the power capacies of ressitors determine there size,materials used,ability to disipate heat. I would use the big square 5watt ceramic ones,no need to take chances with your automobile.

Dr. Pepper

VR1, VR2, and the 10k resistor can be as small as 1/4 watt. You will never exceed 0.02 watts in any of the resistors of the circuit. So even 1/4 watt variable resistors will be fine. There is no harm in using higher wattage components but it is a waste of money and space. There is only one high amperage trunk in the whole circuit. It is the path furthest to the right of the circuit where the +12V goes through the bulb and to the transistor's collector and then to its emitter to ground. The wire on that path needs to be the heavier gauge wire and the transistor needs to be heat sinked. Make sure it stays cool after having the dome light on 4-5 minutes. The only fire hazard or concern is that you keep the 2N3055 transistor from getting blazing hot. If it overheats and shorts the dome light will stay on all the time. You will find the rest of the circuit stays cool without any need for large components or heatsinks.

And yes, simply put hook up the relay coil to your old dome light wires and replace the switch with the relay's normally open contacts. Everything works the same as before except when the dome light goes off it goes off slowly. You should be able to patch the circuit in under the dash once you locate your 2 dome light wires and a place to bolt a ground wire to the body. 1 dome light wire should be +12v from the battery while the other runs to the switches that control it.

Also someone mentioned soldering irons. 15 watt irons are way too small for most soldering. 15 watt irons are only really suitable for surface mount soldering. For general purpose soldering try 33 watts. 25 works but its important that it stays clean and well maintained. Anything above about 33 watts that isn't temperature controlled gets too hot. Also look for irons that do not have a bright chrome looking finish to the tips. I don't know who started that idea but it never works. Radio Shack's modular iron, the one that you buy all the pieces separately, works descently. The only problem with it is the modular design is not very sturdy so don't pry or push with it. Most of the time if you are prying or pushing you are doing something wrong anyways.

junkyarddog

shouldn't the transistor be reserved for the door circuit only?? why would a relay need to be in place of a switch??

89 Iroc Z

I am almost ready to buy all the parts I need but I have a question about the circuit. On the diagram it shows one contact from the 2N3055 transistor going to the VR2 variable resistor but the 2N3055 is a 2 pin transistor, so how can this be? I have built many circuits before but I am no expert so I need just a little extra help.

SgtSkip

The relay is replacing the switch only in the dimmer circuit because our door switch goes to ground, not +12v.

Dr. Pepper

The idea behind using the relay with the current dome light wiring is simplicity. Sure you could make it go off slowly with the doors and turn off instantly with the dimmer control. It just adds complexity and wiring. If the time it takes to fade out is fairly small I can't see it being worth the trouble. If what you wanted was to have the light stay on for some time and then fade out quickly or go off instantly then it probably would need to only work with the door switches. Anything is possible its simply a matter of how hard are you willing to work to accomplish it.

As SgtSkip pointed out the relay handles the conversion between the F-body switch/door system and the one that the circuit was designed for.

The 2N3055 is a 3 pin device, the metal case of the transistor acts as the 3rd pin. So you will have to bolt one wire under a mounting bolt. Take a little caution that the body of the transistor and the heat sink stay clear of anything metal. When you go to Radio Shack the transistor will come on a card which will tell you where each connection is on the transistor.

89 Iroc Z

Thank you "Dr. Pepper" and everyone else for the help,
I was surfing around a few electronics sites and I found every part I need but I am unsure on a few parts. Like the uA741 I found 2 different ones at http://www.digikey.com, the UA741CD and the UA741CP is the only difference between the 2 shapes? Also what type of capacitor should I buy? I am thinking aluminum electrolytic capacitor. Is a 22uF really going to give enough juice to fade out the dome light?

Thanks for everyone’s help

SgtSkip

Radio Shach doesn't have a 741? Thats odd 'cus its a really common opamp. Doc, wouldn't the rate that the cap is discharged be controlled by vr1 and the 10k resistor? Im used to dealing with RF and haven't looked at an opamp circuit in ages!

Petes 84Z28

Why couldn't a person install a capacitor in parallel to the dome light? When the circuit closes (ie: the door opens), the light will come on as normal and the capacitor will also charge. Then, when the circuit opens (door closes), the light will gradually dim as the capacitor discharges?
It may take a bit of 'trial-and-error' to find an acceptable size capacitor. Also a resistor could be installed in series with the capacitor to dim the light immediately when the circuit opens, and then more gradually fade away...
Is this plausible, or am I overlooking something totally?

Pete

SgtSkip

I'm sure you could do that with a large enough cap but the light would act as a pretty large load so it would discharge pretty quickly.

Apeiron

A capacitor won't work, for a number of reasons.

Dr. Pepper

Try an LM741CN. I looked in my parts drawer and I had these so they should be a good general purpose version of the 741 op-amp. I'm too tired to look up the minor differences in the suffixes. This is not a critical circuit so the most important feature is that it is a DIP package IC.

An aluminum electrolytic is not a bad choice. Normally this would be the type of capacitor used. But I might suggest trying P2040-ND. It is a 22uF tantalum capacitor. The reason is that the electrolytics are acid based similar to car batteries. When exposed to heat regularly they tend to not last long. Without tinting or louvers F bodies get very hot in the summer. This will help prevent failures down the road in 5 to 10 years. In mass production the high cost of tantalum is a major concern, but for one circuit an extra dollar is not important. It is your choice. The aluminum electrolytic can be replaced at any time with a tantalum. So if you walk in to Radio Shack to buy everything just grab a 22uF electrolytic. Another note about Digikey, I believe they have an extra charge for orders under $25. So by the time you add in shipping and extra charges it might be cheaper to just go to Radio Shack.

The 22uF is simply a timing capacitor. It could be replaced with a 1uF with minor changes in the circuit. The reason the circuit uses an op-amp is to let the op-amp mirror the discharge of the capacitor without the capacitor seeing a load other than the 10k resistor and VR1. As asked, these are what dictate the discharge rate of the capacitor.

I can't think of any reason why a simple parallel capacitor would not work. So if anyone has a reason why it wouldn't please don't keep it to yourself. It is a bit of an ugly solution however. The capacitor would have to be very large. Charging such a large capacitor would most likely stress all of the switches in the circuit. I am not certain a large enough capacitor is readily available. Any current limiting resistors used would also have to be very large to prevent overheating. The best plan is to stick to a circuit similar to this one.