# Need help with a circuit

Discussion in 'Electronic Basics' started by Zinfari, Aug 19, 2004.

1. ### ZinfariGuest

I have a circuit that requires a 10k pot to function correctly. The problem
that the device that has to be used has a 5k pot and it can't be changed.
How would I go about getting this circuit to work properly? I still need
full range so basically I need a 2x multiplier on the 5k pot. Thanks in

-Zinfari

2. ### Peter BennettGuest

Depends on how the pot is used in your circuit.

If the pot is used as a voltage divider (all three terminals used),
the 5K pot will probably work OK as is.

3. ### ZinfariGuest

No only 2 are used that's my issue. I need (roughly) 0k at start, 5k at 1/2
turn and 10k full so a simple resistor in series or parallel won't work
either.

Anyone have any ideas?

-Zinfari

4. ### Michael A. CovingtonGuest

Depending on the circuit, you may not need any change at all. What is the
circuit?

5. ### Robert C MonsenGuest

You can use an opamp and NPN transistor to emulate a 10k pot using a
5k pot.

VCC
+
|
.-----.
| |
'-----'
|
.-------------------------o V1
| |
.-. |
| | |\ |
R | | +------|-\ |/
'-' | | >--------| NPN
| | +---|+/ |>
o----+ | |/ |
| +-----------------o V2
| .-.
.-. | |<---,
R | | 5k | | |
| | '-' |
'-' o-----.
| |
'-------------------------o
|
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

This circuit makes V1 = 2 * V2. Thus, it acts like a 10k pot. As the
resistance goes from 0 to 5k, the total resistance of the NPN + 5k
goes from 0 to 10k. It'll freak out at low settings of the 5k pot,
because it can't make the voltage across the NPN get less than a few
tenths of a volt.

Regards,
Bob Monsen

6. ### John FieldsGuest

---
You're a fucking idiot.

Oh wait...

Hopefully you're _just_ an idiot. That way you won't be able to
figure out how to procreate and we won't have to deal with the issues

You mention nothing about about what the input looks like and you
mention nothing about what you want the output to look like, yet you
whine about what you'd like us to do for you when you apparently have
no idea about what you need.

But, this _is_ sci.electronics.basics., so I apologize for any
aspersions I may have cast on you and I'll, instead, beg you to ask
for what you want more clearly so that I might serve you in a fashion
which will make you less uncomfortable...

7. ### petrus bitbyterGuest

A solution largely depends on the circuit and the function of that pot in
it. A universal - and impractical and expensive - solution can be to use the
5k pot as a voltage divider, use the voltage to control a servo, which in
turn controls a 10k pot. For a better solution we - at least I - need more
information.

petrus bitbyter