Basic electronics help

maxxell · 05-26-2005, 04:34 PM

Hey all,
I decided to learn some rudimentary robotics this summer as a hobby. I'm starting from square one, I never took a class on electronics or anything, so I bought a beginner's book.

Everything is going swimmingly, but I have a question I was hoping someone here could help me with.

Here are the various stages of the circuit I'm working on:

1.
9v -> SPDT switch -> resistor -> LED -> 9v
LED lights up

2.
9v -> SPDT -> resistor -> trimpot (little wheel to vary resistance) -> LED ->9v
LED lights up, and brightens / dims with the trimpot

3.
9v -> SPDT -> resistor -> trimpot:
trimpot 1-> LED1 -> 9v
trimpot 2-> LED2 -> 9v
LEDs light up, trimpot dims one, brightens the other, and vice-versa

4.
9v -> SPDT -> resistor -> trimpot:
trimpot 1-> photoresistor1 -> photoresistor2 -> LED1 -> 9v
trimpot 2-> photoresistor3 -> photoresistor4 -> LED2 -> 9v

Now the trimpot doesn't seem to do anything. Spun all the way to either extreme doesn't affect the brightness of the LEDs. The armount of light the photoresistors receive DOES vary the individual LED brightness, without affecting the other LED.
The only way I could get the LEDs to vary in response to the trimpot as it did in version #3 was to use a bigger trimpot.

Everything works as I would expect with a larger trimpot. Why does the addition of the photocells necessitate using a larger (more resistance) trimpot?

Thanks!

Vee24 · 05-26-2005, 05:23 PM

Sorry I can't help you, but what book is that? By whom?

maxxell · 05-26-2005, 05:33 PM

Robot Building for Beginners. It's by David Cook. I got it at Borders.

migel628 · 05-26-2005, 05:55 PM

Sorry, I can't help either...ISBN please

maxxell · 05-26-2005, 06:27 PM

Quote: Originally Posted by migel628

Sorry, I can't help either...ISBN please

Tell you what... Send me some cash, I will buy the book, learn its tenets, build the robot, and send it to you.

maxxell · 05-26-2005, 07:03 PM

A friend of mine thought it would be illuminating to tell you all the details about the parts.

Parts List:
9V battery
SPDT switch
470 ohm resistor
Trimpot: 20 kilo-ohm variable
2 red LEDs

4 photoresistors
1:
Dark (held under table) = 47ko (kilo-ohm)
Light (held 1" from 60W light bulb) = 174 ohm
2:
D: 77 ko
L: 100 ohm
3:
D: 46 ko
L: 162 ohm
4:
D: 75 ko
L: 120 ohm
[Chosen to roughly give the same total resistance in both dark and light extremes. 1&2: D: 124ko L: 274 ohm 3&4: D: 121ko L: 282 ohm]

mp3hombre · 05-26-2005, 07:16 PM

Could you post a schematic? Perhaps is the wiring that needs to be changed.

maxxell · 05-26-2005, 07:40 PM

I forgot to include the switch... it's above the first resistor.
PR = photoresistor

migel628 · 05-26-2005, 07:42 PM

Quote: Originally Posted by mp3hombre

Could you post a schematic? Perhaps is the wiring that needs to be changed.

A schematic would be the best route to help us help you. Maybe it'll jog my old electronics memories

Edit: Beat me to the post...Sweet

Edit2: I'm at a loss man, sorry...

migel628 · 05-26-2005, 07:52 PM

Quote: Originally Posted by maxxell

I forgot to include the switch... it's above the first resistor.
PR = photoresistor

Actually, I thought a little more on it. It looks like the LEDs are getting voltage from both 9v sources. Once you add a larger pot in the mix, you're effectively cutting out one 9v source, resulting in the dimming you would expect from your example 3. Basically, you're doing the job of the photoresistors w/o actually using them. If anyone has a better explaination, then feel free to correct me...

maxxell · 05-26-2005, 08:06 PM

Both 9v sources?
Maybe it's my shoddy schematic-making. There's only one 9v battery in there. The 9v at the top is the positive terminal and the 9v at the bottom is the negative terminal.

mhornnt · 05-26-2005, 08:19 PM

based on that schematic, adjusting the trimpot should brighten/dim the leds. increasing its resistance is gonna drop the voltage across the LEDs, which will dim them, and the opposite in the case of lowerin its resistance. Only think i could think of that might cause you not seein difference as you adjust it is that its resistance value is so small compared to the photo reisistors that adjusting it just is only creating a negligible change in the voltage across the diodes, small enough that you arent seein a change in brightness.

migel628 · 05-26-2005, 08:20 PM

sorry, it looked like you had 2 9v sources in the schematic

mp3hombre · 05-26-2005, 08:36 PM

First of all, the photocells (photoresistors) should be the same type. Their resistance in dark conditions might be too high, the leds are not getting enough "juice" no matter how much you spin the pot.
To bring the resistance down, try adding a resistor across PR1 - PR2 or across each photocell as in PR3 and PR4. Start with 20Kohms.
Experiment, experiment!! And keep us posted.

maxxell · 05-26-2005, 11:06 PM

Why does adding a resistor in those places reduce the resistance to the LEDs?

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