Re: Need help to extend capabilities of a simple proximity circuit
- From: "Bob Monsen" <rcmonsen@xxxxxxxxx>
- Date: Sat, 26 Apr 2008 08:33:00 -0700
"axrock" <chris.sefton@xxxxxxxxx> wrote in message news:2c1e95f4-5659-4188-a918-1c31fbda9a6e@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi,
I am a total rookie when working with circuits, but I have built a kit
set transonic proximity sensor. I'll explain my use at the end.
Essentially what it does is turn on an LED when an object gets near to
the sensors. I have extended the kit with another kit to add relay
functionality. So when an object nears the sensor it switches the
relay. All this is a simple kit set circuit that runs on 12V.
My only problem with it, is the sensors a very touchy, when it senses
an odd shaped object (like a human or dog etc) the relay switches
madly on, off, on, off until the object is really close (EG: 2 Inches
etc).
I am wanting to add a delay to the circuit, so that when the relay
switches it stays on for at least 3 or 5 seconds. In other words, when
the sensor begins to detect the presence of an object rather than the
relay going nuts, it will actually stay on for at least 3 or 5 seconds
etc. When it is due to turn off again it will likely receive another
erratic pulse from the sensors due to the odd object near by. Hope
that makes sense.
What this is designed for:
Just in case you are curious.
I am attempting to build a device that will allow my dog to turn on an
outside tap to allow him to drink straight out of the hose. This way
he always has fresh water whenever he needs it. So far I have a
washing machine solenoid which switches on a home made pressure mat.
This does not work that well though. Hence, using a proximity sensor.
My dog just needs to walk up to the water outlet and the water turns
on. It works at the moment, but switches on and off very fast as he
gets within the distance I want. I need to set a timer of sorts
(possibly a 555) to keep the relay on rather than switch it off
immediately. This way it should keep the water flow steady for the
drink, and will turn off within 3 or 5 seconds of the dog moving away.
Really appreciate any help.
Really just need some help to add a small delay timer between the
sensor circuit and the relay circuit.
Thanks,
Chris
The sensor probably puts out a voltage level which depends on proximity. That voltage level is tested by a 'comparator' circuit to see if it is high enough. If it is, it turns on the output. The voltage at which it decides to turn on the output is a switching point.
The problem is that the output is not steady, but has a certain low-level noise. As the dog is getting closer, the noise causes the thing to trigger on and off until the dog is close enough so the noise + signal is no longer below the switching point.
The way this is typically handled is to use some positive feedback from the output to change where the switching point is.
Say the thing switches at 6V normally. If you put in some positive feedback from the output, once it switches on, the switching point will go down to, say, 5V. If the noise is less than a volt, it will stay on and not chatter.
If the dog is satisfied, and is leaving, the voltage will begin to decrease, but the water won't go off until the output + noise gets down to 5V. At that time, however, with the positive feedback, the switching voltage will be modified to 7V, so it won't chatter. Now, when he returns to get more water, it won't switch until the voltage gets to 7V.
In order to determine how to do this, you need to post the circuit you built, so I can see where to put the extra resistor.
The other problem you might be encountering is that the relay coil takes a certain amount of current to switch the relay on. If your sensor is measuring a voltage, it might be affected when the coil turns on and off. Does the mad switching occur when the relay is not hooked up? If not, then you can try putting a big electrolytic capacitor (like the ones they use in power supplies), like 3300uF, across the sensor power inputs, to protect it from dips when the relay turns on.
You can also run into this problem with a 555, which can end up either not turning on, or staying on and not shutting off, due to the change in the 12V input.
Fun project.
Regards,
Bob Monsen
.
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