# In need of a simple circuit.

Discussion in 'The Projects Forum' started by Mutad0r, Jan 17, 2011.

1. ### Mutad0r Thread Starter New Member

Apr 6, 2010
10
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I'm a noob at this...

All I need, is to power 4 objects from a same source, if they all needed the same amount of current, I'd probably just put them in series, and leave it at that, but almost all of the 4 objects are different. I have tried this for about a month, but I can't seem to understand the ohms law ( among other things) ... I would appreciate it if someone was to provide me with a circuit for this incredibly simple thing. Further more, I would learn something from it.

The objects I have:
Laser: 4,5v 150 mA
two motors: 1,5v 250 mA (Each)
smaller motor: 1v 50 mA

I have all sorts of adapters available, so I just use whatever power source you thing fits best. The objective is to have all of those 4 objects working without smoke coming from any of them (within the month, I have managed to burn out a laser =[ )

2. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
Do not put things like this in series. Make 3 regulators for the various voltages (or two from lower voltages) and put them in parallel.

3. ### Mutad0r Thread Starter New Member

Apr 6, 2010
10
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Progress.. But not enough for MY brain. First time I hear of these reg-uh-lators.. (I'm not a native English speaker.. AND I'm rather new to "this" )... I'll try google, but I doubt it will instantly make sense to me..

4. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
719
You'll want to limit the current to the laser with a resistor, treat it as an LED (laser is probably an LED based type, so it's the same).

Use separate power regulation for each output, I'd strongly suggest AGAINST using any "voltage divider" regulators, as the voltage changes with the load, in addition to wasting a good deal of power.

Trying to get the 1V regulated will be difficult. The 1.5V is easy enough with a single battery.

Actually, all of the supplies are simple with batteries.

Laser - 4AAs in series + current limiting resistor.
1.5V Motors - AA battery each
1V Motor - AA Battery + Silicon diode in series.

5. ### wayneh Expert

Sep 9, 2010
13,435
4,272
+1

Except, if the two 1.5v motors are identical, it wouldn't be crazy to put them in series and use a 3v supply. It would still be better to put them in parallel with a 1.5v supply.

The battery suggestion is a good one. The diode mentioned provides a fixed drop of ~0.6v, giving just under 1v from a 1.5v battery.

C'mon back if you need a circuit instead of batteries.

6. ### Mutad0r Thread Starter New Member

Apr 6, 2010
10
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Batteries seem to be the easiest way to go... But I think I'll go with the circuit.

7. ### wayneh Expert

Sep 9, 2010
13,435
4,272
I think the approach should be to design two supplies, one for the laser and one for the motors. The supply for the motors can be a 1.5v regulated supply, again using the diode trick to drop the voltage for the 1v motor.

I'm not familiar with building a 1.5v supply - that voltage is too low for me - but someone here probably knows how.

The supply for the laser is a bit trickier. Assuming it's like an LED, I think it should actually be a current-controlled regulator instead of voltage-controlled. How certain are you of the power demand? Can you post more details about your laser?

8. ### Mutad0r Thread Starter New Member

Apr 6, 2010
10
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Out of frustration, I "accidentaly" burnt out the laser I had intended for it. So instead I salvaged one from a laser pointer. It had 3 button cell batteries powering it. I googled them, and wikipedia said they were 1,5 V and 150 mA each.

9. ### wayneh Expert

Sep 9, 2010
13,435
4,272
Well yes, but that's the max current, not necessarily what the laser is really using. But anyway that sets the upper limit of what the battery replacement must provide. Any other current control is handled within the circuitry of the laser pointer itself (which you're still using, right?), so we're back to a voltage regulated 4.5v supply. That's great if we don't have to be concerned with current control to the laser.

A 5v USB charger with a voltage dropping diode would work fine, or you could build your own 5v supply using a transformer+7805 IC+ the diode. If the laser isn't too expensive, I'd just try using a 5v supply without the diode. I doubt the extra voltage would really be a problem.

10. ### Mutad0r Thread Starter New Member

Apr 6, 2010
10
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Yes, I kept the tiny circuit that came with it.

I'm trying to make as much sense of this as I can, but I'm not being very successful. Could you draw a primitive schematic of What this all is supposed to be?

11. ### wayneh Expert

Sep 9, 2010
13,435
4,272
Do you have any USB-type 5v supplies (small, light), or are we starting with old-style transformer wall warts (heavy, squarish)?

If you have the USB-type supply, can you sacrifice it (cut off the end connector) or do you have the ability to otherwise break out the power pins (opposite sides of the standard USB connector)? Adding the diode is as simple as orienting it correctly and putting it in line with your laser on either lead.

If your power source is unregulated, ie. the typical old wall wart, we'll have to look at the 7805 IC for creating a regulated supply from it.

12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
719
Going off the specs of a Common 5mW Red Laser, Threshold current is 18-25mA, operating current is 20-25mA, I've seen more references to 20mA than to 25mA, and anywhere from 2V - 4V.

So clamping the current at 20mA would do the trick for the laser, more than likely. A small increase in current will not significantly increase the power, but will cause the beam to be less coherent. There may be a sweet spot around 22-25mA if this is a 5mW laser element. A part number from the burnt out one would help.

Most likely, there is a tiny grey/black resistor on the board where the LED Laser is mounted, dimensions are in the mm range, e.g. hard to even spot.

Getting the value of that resistor may help.

13. ### Mutad0r Thread Starter New Member

Apr 6, 2010
10
0
I don't have any USB type supplies... however, I have about a million ( more like 40 actually) power adapters ( what you seemed to be calling a wall wart )

As for the laser, the resistor is too small for me to make anything out of it, however 25 mA seemed best for the laser.

EDIT: I just remembered, I DO have a microscope.. it says "620" on the resistor.

Last edited: Jan 19, 2011