# Comparator? or maybe not?

#### jimbo1954

Joined Sep 23, 2007
3
I'm a network engineer, and know virtually no basic electronics. However, I need to make a small circuit for a piece of kit I'm modifying, and I could use some advice!

I have a direction sensitive output from an accelreometer, so that 0g is indicatd by 4v output, 1g leftwards is indicated by 0v output, and 1g right is indicated by 8v.

I want to be able to take that voltage and convert it so that I get a left 0-1g output of 0-4v on one wire, and a right 0-1g output of 0-4v on another wire.

I guess what I need may be called a comparator, though I'm not sure, or it may be a kind of op-amp. Problem is, without having a name for it, I can't google it

Can anyone put a name to the kind of circuit I'm describing, and maybe provide a circuit diagram

Thanks

Jim

#### bloguetronica

Joined Apr 27, 2007
1,541
A set of two comparators will do. An LM393 chip (or the quad version LM339) will be suffice. It is always a good idea to use comparators. You just need one to evaluate if the voltage is greater than 4V, and the other one to evaluate if the voltage is greater than 8V. You might need additional logic gates to evaluate the results.

Give details of what kind of output you want to use. How should it behave? Let me know if I'm giving you the adequate sugestion.

P.S.: The LM324/LM358 op-amps would give comparators too, although they are not specially designed to do so.

#### jimbo1954

Joined Sep 23, 2007
3
Thanks for the quick response. What I have in mind is to take the absolute output from a solid-state accelerometer, and condition it as necessary through an op-amp to get the correct power to drive the downstream circuits.

Then I need to feed the resultant signal into the "signal separator" I described before, so I canuse the two outputs to drive a pair of LM3914 Bargraph Display circuits, so I can get a bargraph display for left g-forces and a bargraph display for right gforces.

#### recca02

Joined Apr 2, 2007
1,212
from what i am able to understand about your requirement u might be required
to use two diodes(in different circuits connected in opposite directions) after using the comparator with 4v as a reference so one diode will conduct when o/p is negative and other when o/p is positive(the circuit whose diode is forward biased for +ve o/p from comparator will give right g force acceleration signal)
i think the comparator gain should be set to unity for this purpose.
ofcourse there must be a better wa to achieve the above isolation of signals.

#### jimbo1954

Joined Sep 23, 2007
3
I can see where you are driving with the "two opposed diodes" approach, feeding power between 0-4 volts one way, and 4-8 volts another way...can you suggest an example diagram so I can be sure that I understand what to do and do not generate sparks and flames

Thanks!

Jim

#### recca02

Joined Apr 2, 2007
1,212
i am really no good with circuit design,i might miss out on various rating and stuff (since i do not have practical experience with electronics i m here just to learn electronics myself--i am more of a mechanical student),
like i mentioned someone here better than me might help with this (if not as a last resort i will post a really pathetic ckt).
BTW i think u got what i intended to say.
the comparator will o/p voltages below 4v as negative and above as positive.
so the diodes might just do the trick i;e one will conduct only for postive and other for negative hence we achieve isolation of the signals and since the voltage levels are quite low i dont think u shud worry about frying anything.
one drawback about this idea is that there will be a drop of abt .7v across the diode and it will also act as a dead band--so not really a very good idea.

#### Ron H

Joined Apr 14, 2005
7,063
This should work. It's not comparators, it's op amps. There may be a simpler way to do it.
V1, the -4V battery, is actually a voltage reference, and it could have another value, so long as V1/R3=400uA. For op amps, try TL074 or something similar.
U1 inverts the input and level-shifts it to be symmetrical around zero volts. Full scale (FS) left at the U1 output is +4V, FS right is -4V.
U2 reinverts the signal. FS left at the U2 output is -4V, FS right is +4V.
U3 and U4 are half wave rectifiers that clip off the negative portions of their respective iputs.
U5 and U6 are voltage followers, to give you a low output impedance that will drive a light load without error being introduced by the load.

#### Attachments

• 20.4 KB Views: 21
• 26.8 KB Views: 18