556 Trigger Voltage Question

[Terry]

Hacked a motion detector for a Halloween prop. Shows 24 volts when I
measure the two wires I soldered on the detector circuit board. When
the detector detects someone, the voltage drops to zero.

Put together a standard non-retriggerable timer circuit using a diode
off the second output to control the non-triggerable duration per
responses to a previous message. Running this circuit at 9 volts.

When I fasten the positive from the detector directly to the triggers
and the negative to the common ground of my circuit, it reads 24 volts
until something detected and then drops to only 7.5 volts. This is not
enough to set off the timers.

Not being an electrical genius, can someone explain to me why the drop
to 7.5 volts instead of 0 when I hook the detector to my circuit?
What's the best way to hook up this detector to the circuit to trigger
the timers?

Thank you for any help.

Terry

 

[Terry Pinnell]

Hacked a motion detector for a Halloween prop. Shows 24 volts when I
measure the two wires I soldered on the detector circuit board. When
the detector detects someone, the voltage drops to zero.

Put together a standard non-retriggerable timer circuit using a diode
off the second output to control the non-triggerable duration per
responses to a previous message. Running this circuit at 9 volts.

When I fasten the positive from the detector directly to the triggers
and the negative to the common ground of my circuit, it reads 24 volts
until something detected and then drops to only 7.5 volts. This is not
enough to set off the timers.

Not being an electrical genius, can someone explain to me why the drop
to 7.5 volts instead of 0 when I hook the detector to my circuit?
What's the best way to hook up this detector to the circuit to trigger
the timers?

Thank you for any help.

Terry

Can't say without seeing your schematic. In particular, you've not
stated what supply voltage you're using for the timer circuit. I'm
assuming not 24V, which would be beyond the 555's capability.

Possibly you're not edge-connecting the trigger(s)? IOW, using a
capacitor to isolate the DC voltage level and just pass the negative
edges to the 555 trigger(s). A simple example is shown here:
http://www.terrypin.dial.pipex.com/Images/555OneShot.gif

If correctly configured, a brief 24V to 0V pulse from your motion
detector should satisfactorily trigger a 555 mono using a lower supply
(e.g. 9V). BTW, to be on the safe side, I'd also connect a diode from
the 555 trigger(s) to the +ve supply. (I assume you *have* connected
the grounds of both sections?)

If you post/publish your schematic, we can probably be more helpful.

 

[Fred Bloggs]

Hacked a motion detector for a Halloween prop. Shows 24 volts when I
measure the two wires I soldered on the detector circuit board. When
the detector detects someone, the voltage drops to zero.

Put together a standard non-retriggerable timer circuit using a diode
off the second output to control the non-triggerable duration per
responses to a previous message. Running this circuit at 9 volts.

When I fasten the positive from the detector directly to the triggers
and the negative to the common ground of my circuit, it reads 24 volts
until something detected and then drops to only 7.5 volts. This is not
enough to set off the timers.

Not being an electrical genius, can someone explain to me why the drop
to 7.5 volts instead of 0 when I hook the detector to my circuit?
What's the best way to hook up this detector to the circuit to trigger
the timers?

Thank you for any help.

Terry

You don't think it's important to tell us what you have hanging off the
555 TRIG input besides that diode? Whatever it is, it's loading the
motion detector output with too much current. If you are using the
standard 555 , then the diode is all that you need- you can run the
24VDC line directly through the diode to pin 2 TRIG of the IC and
nothing else (no pullup to Vcc required)- it makes no difference what
power supply you use for the 555 just so long as it is less than 24V.
This will cause the 555 monostable to output a pulse that is the longer
of the motion detector and the 555 RC network. You do notice that the
diode has a *band* on it and this is the cathode which connects to the
motion detector.