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'[PIC]:Newbie Circuit Question'
2001\04\04@170307 by kben

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Hello Everyone,
I have a question concerning a circuit I am building. I am building a simple
stoplight controller. I have a 9v 500ma wall wart supplying power to a 7805
regulator.
The regulator powers the 16f84 Pic. The portb pin is connected to a 1K resistor,
that is connected to the base of a darlington transistor. I have a 12.8v 1.04A
light bulb connected between the unregulated 9v 500ma supply and the collector
of the transistor. The emitter is then connected to ground.
The code just sets one pin(portb3)high and the other one low(portb4), and then
reverses the process. The problem is the second bulb will not light unless I
disconnect the first bulb. Then I reconnect the first bulb and the circuit works
fine. Any suggestions for circuit improvement. Obviously I am new to electronics,
but I am comfortable with programming.

TIA,
Kevin

PS. I got the circuit off the net. It called for a BD679, but below is the closest
I could find.
Transistor Specs
pwr disp 65w
coll-emitr voltage 60v
coll-base volt 60v
emit-base volt 5.0v
coll current continuous 5.0A
coll current peak 8.0A
base current 120ma

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2001\04\04@190025 by Tony Nixon

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"Kevin A. Benedict" wrote:
>
> Hello Everyone,
> I have a question concerning a circuit I am building. I am building a simple
> stoplight controller. I have a 9v 500ma wall wart supplying power to a 7805
> regulator.
> The regulator powers the 16f84 Pic. The portb pin is connected to a 1K resistor,
> that is connected to the base of a darlington transistor. I have a 12.8v 1.04A
> light bulb connected between the unregulated 9v 500ma supply and the collector
> of the transistor. The emitter is then connected to ground.
> The code just sets one pin(portb3)high and the other one low(portb4), and then
> reverses the process. The problem is the second bulb will not light unless I
> disconnect the first bulb. Then I reconnect the first bulb and the circuit works
> fine. Any suggestions for circuit improvement. Obviously I am new to electronics,
> but I am comfortable with programming.

It sounds like a simple maths problem to me.

Wall wart = 500mA

1 light bulb + 1 light bulb = 2.08 Amps


500mA - 2.08A = - 1.580A

Well, it doesn't exactly work like that, but you are trying to draw more
current from the wall wart than it can supply. Consequently, it's output
voltage is dropping down to a point where the regulator fails to work,
and also the PIC.

Try using a power supply that can supply the required current, or use
bulbs that draw less current.

Also, if you use LEDs during code development, there is less chance of
damaging the circuit and PIC.


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Tony

mICros
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2001\04\05@013851 by Roman Black

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Kevin A. Benedict wrote:
{Quote hidden}

1. What number is on the transistor? This does
not sound like a darlington.

2. You cant run 2x 12v 1A bulbs from a 9v 0.5A
wall wart supply. It will not supply enough amps.
:o)

3. Can you show the circuit?

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2001\04\05@052506 by kben

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OK, point taken about the 500ma power supply.

>1. What number is on the transistor? This does
>not sound like a darlington.
I am at work, but does TIP120 sound right. It says darlington on the package.

I got it from Radioshack.
>2. You cant run 2x 12v 1A bulbs from a 9v 0.5A
>wall wart supply. It will not supply enough amps.
It is a stoplight configuration, so only ONE bulb is lit at a time
loop:
BSF RED
BCF GREEN
DELAY 2 seconds
BCF RED
BSF Green
Delay 2 Seconds
goto loop

>3. Can you show the circuit?
http://www4.tpg.com.au/users/talking/25mA.html
It's the fourth circuit down, using the 12c508a

The circuit does work if I put a 1.5k resistor for the second bulb. Bulb 1 blinks
by itself for two seconds, then it goes into the loop of red then green then
red then green ?
I realize I should just get the proper 1A power supply!  :)

Thanks,
Kevin

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2001\04\05@053538 by Roman Black

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kben@dca.net wrote:

> I got it from Radioshack.
> >2. You cant run 2x 12v 1A bulbs from a 9v 0.5A
> >wall wart supply. It will not supply enough amps.
> It is a stoplight configuration, so only ONE bulb is lit at a time
> loop:

> The circuit does work if I put a 1.5k resistor for the second bulb. Bulb 1 blinks
> by itself for two seconds, then it goes into the loop of red then green then
> red then green ?
> I realize I should just get the proper 1A power supply!  :)

You bet!! Remember too that a 1A bulb will draw a short
surge of a LOT more than 1A before the filament heats.
Use a big cap across the power rails, probably 2200uF
25v sounds good, more is better.

Also, a trick suggestion, allow 0.2 seconds where
both bulbs are off before you turn the next bulb on,
this will allow a slight break before the next turn
on surge. You won't notice this with 2 sec on periods.
-Roman

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2001\04\05@065410 by David Duffy

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Kevin wrote:
>OK, point taken about the 500ma power supply.
>
> >1. What number is on the transistor? This does
> >not sound like a darlington.
>I am at work, but does TIP120 sound right. It says darlington on the package.
>
>I got it from Radioshack.
> >2. You cant run 2x 12v 1A bulbs from a 9v 0.5A
> >wall wart supply. It will not supply enough amps.
>It is a stoplight configuration, so only ONE bulb is lit at a time
>loop:
>BSF RED

Insert at least a couple of NOP's here

>BCF GREEN
>DELAY 2 seconds
>BCF RED

And here too.
You might be getting bitten by the infamous read-modify-write bug!

{Quote hidden}

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U8, 9-11 Trade St, Cleveland 4163 Australia
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2001\04\05@130845 by Barry Gershenfeld
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>>OK, point taken about the 500ma power supply.

The cheap and easy way out, would be to just go buy another
wall-wart and run the lights off it separately.  That way
your lights will be kind of "what you see is what you
get" (which is what  you were thinking originally), but it
won't cause the processor to reset.   Tie the grounds
(the negative leads) of the two supplies together.

For a resetting processor...always ask yourself if what
you are seeing would be caused by the program starting
over and over again.  In your case, yeah.

 do {
       turn on the first light;
 } while(1);

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