[sdiy] Voltage-limiting a comparator output.

[Tim Parkhurst]

On Sat, Aug 27, 2011 at 1:56 PM, Dave Kendall <davekendall at ntlworld.com> wrote:
> Hi all.
>
> I'm working on a circuit that aims to accept a gate anywhere between ±15V,
> and convert it to a positive voltage between 0V and +5V, for driving logic
> circuits.
> The first bit seems simple enough - an op-amp based comparator, followed by
> a series diode to give half-wave rectification.
> Then I had thought of feeding the comparator's output into a simple voltage
> divider, to bring the output down to +5V.
>
> Any suggestions welcome.
>
> cheers,
> Dave
>

How about just using a transistor? Comparator output to resistor (50k
or so) to base (add a diode to ground to block negative pulses going
to the transistor). Tie the collector to +5V, tie the emitter to
ground through a 500 Ohm resistor, tap the output from the emitter.
Won't this work just like an open collector output (like the LM339
suggested earlier) or am I being typically dense?

Otherwise, the TI 7726 is pretty cheap (6 circuits for less than $3 or
about 50 cents per circuit). Easiest seems to be a zener with a series
resistor to limit the current.


Tim (any denser and I'll start trapping small animals in my
gravitational field) Servo
-- 
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings