I'm creating some circuits required in a student-built electric-vehicle application. There are no heavy loads on the low-voltage +12V system.
What I have below is a circuit that does some threshold detection (LEFT), which indicates that status to an isolated output-circuit (RIGHT).
The intention here is that
U3 (LM555) and
Q2 (IRF9540N) should act in a complementary way to each other:
I've tested this output circuit using a relay to switch
SIG to ground, and that works well!
In attempting to iterate a solid-state-only design, I attempted to replace the relay with darlington optocoupler
U2 which produces the following effects:
SIGis pulled HIGH (
U3is active and
Q2is OFF - as expected.
U1triggers, driving optocoupler
SIGis not pulled entirely to ground - rather, about 0.5V as measured with a multimeter. This appears to be enough to (partially) activate both
Q2is clearly on because Vgs is much less than Vgs(th). What's curious is that
U3appears to drive LOAD-B on.
Does this imply that there is some oscillation in
SIGor at least at the ENABLE pin of
U3? I would have expected the chip to be either active (oscillating) or not. Not some intermediate state.
I have a few ideas on how to improve this circuit, but was wondering what readers here might think.
Specifically, my questions are:
- Is this a naive approach to creating a voltage-robust input in the right-hand circuit? I want to protect somewhat against noise and wiring mishaps.
- Is the 0.5V an artefact of some minimum Vce that is possible across the optocoupler? Remember, substituting a relay or switch works perfectly. I'm a lot more comfortable working with MOSFETs than BJTs (and therefore optos) - this is honestly my first time trying to use one.
- Is a viable solution to schmitt-trigger on the right-hand side's input to create a purely digital signal, and therefore create a strictly complementary operation of
Note: supporting components for
U3 555 timer now shown to avoid ambiguity.