Re: 556 Trigger Voltage Question
From: Terry Pinnell (terrypinDELETE_at_THESEdial.pipex.com)
Date: 10/24/04
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Date: Sun, 24 Oct 2004 12:09:19 +0100
t2hst@yahoo.com (Terry) wrote:
>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.
-- Terry Pinnell Hobbyist, West Sussex, UK
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