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Thread: 12V light dimmer using a 555 timer

  1. #1
    Unenlightened
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    Default 12V light dimmer using a 555 timer

    Has anyone ever built either of these light dimmers http://control.gsnu.ac.kr/~f007dan/c...htm#dimmer.gif or http://www.electronics-tutorials.com...ght-dimmer.htm ? I've made them both from parts available from my local Radio Shack. Neither of them dim a 20W halogen bulb. They seem to run at full intensity no matter where the pot is. I know that without seeing my PCB layout it will be hard to tell what I did wrong. Any advice on component selection would be a great help as I suspect that I'm using the wrong type of capacitors. I'm trying to extend the run time of a bike light and I like the infinite adjustability of the pot. In the meantime I'll try to find a way to get my PCB layouts up.
    Thanks

  2. #2
    Flashaholic*
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    Default Re: 12V light dimmer using a 555 timer

    I have never seen a 555 used quite in this fashion, very cool. Everything looks right in the schematic. I would use a metal film or ceramic capacitor for your tank circuit, that will have better long-term stability and lower ESR than an electrolytic. At that capacitance, they are also quite small. Omitting the upper 1K resistor by accident will damage the circuit, at the very least it will smoke your pot. All I can say is double-check your layout against the schematic. A couple of other design suggestions include the use of a reverse-bias shunt diode across the load, that way any inductance, no matter how small, won't upset the timer circuit. It's a requirement on anything massively inductive such as a motor load or a relay, to keep from destroying your output stage. You can also connect a small (.01~.1uF) bypass capacitor between pin 5 and ground to help stabilize the circuit.

    Though this has absolutely nothing to do with the dimmability, have you looked at using one or more (in parallel) MOSFET transistors to drive your load? Typically that would be more efficient than a bipolar or even a darlington stage. Use 10-100K gate resistors in parallal with a .01uF capacitor. The cap helps shape the pulses driving the MOSFET and will improve efficiency. Hope at least some of this helps!

  3. #3
    Flashaholic
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    Default Re: 12V light dimmer using a 555 timer

    I have built the circuit you are referring to. Something you might try first, replace the light bulb and drive transistor with a LED and limiting resistor. Place a 470 ohm resistor in series with your LED and connect them between pins 3 and the positive supply (+12v). This will give you a safe way to test the circuit and to get a feel for how it works. If it still does not seem to be dimming with adjustment try a small speaker with a 4.7 micro farad cap in series in place of the LED and series resistor. This should produce a tone, if not double check you circuit and try another 555 chip. I usually place a .01 micro farad cap between pin 5 and common as shown on one of the circuits but not the other.

  4. #4
    Flashaholic*
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    Default Re: 12V light dimmer using a 555 timer

    The LED trick will work, try that. However, if you connect between +V and pin 3 its state will be inverted from the desired output--bright will be dim and vice versa. If you connect the speaker, use the capacitor in conjunction with a 330 ohm resistor so you don't overheat the voice coil or damage the 555. You could probably also use a piezo speaker connected directly across the output and ground. If all else fails, you can use an averaging DC voltmeter and it will appear to show you a varying voltage according to the duty cycle--the old fashioned analog type work best for this applictaion. Let us know if any of this helps!

  5. #5
    Flashaholic*
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    Default Re: 12V light dimmer using a 555 timer

    The 555 makes a very poor dimmer, I posted an article about using 3/4 of a LM324 as a variable PWM circuit on here - and it is very reliable and linear

  6. #6
    Flashaholic*
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    Default Re: 12V light dimmer using a 555 timer

    Another circuit that works well is involves two CMOS inverters, such as 1/3 of a CD4049. I have used this circuit successfully and it works very well. Simply build an astable oscillator with two inverters, and in place of the timing resistor use a potentiometer where the end terminals are connected together through a pair of diodes...one to, one from. Use the wiper and the terminal created by the diodes as your pot connections. I would show you a schematic, but I don't have one handy. This too is a very linear circuit, and benefits from very low power consumption (depending on resistor values).

    You can use the rest of the package in parallel to drive your power switching device.

  7. #7
    Unenlightened
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    Default Re: 12V light dimmer using a 555 timer

    I'll try the LED trick as soon as I get a chance. How would you install the shunt diode across the load? I tried to put two diodes in between both leads of the light and the circut and it didn't seem to do anything. To test it with an osciloscope should I hook it up before the MOSFET or after? Thanks for the help.
    The word newbie under my name should be in bold capital letters.

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