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Beam breaker not triggering relay

Discussion in 'General Electronics Discussion' started by QwertyTSecond, Sep 21, 2011.

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  1. QwertyTSecond


    Jul 31, 2011
    I'm currently constructing the coil activation circuit for a coilgun, by using an IR LED and a phototransistor to trigger a relay when the projectile passes through the beam.
    This is the circuit I'm using (minus the IR LED + resistor):

    In addition, I currently don't have any 100K resistors or pots, so I'm using 10K instead.

    I'm finding the coil refuses to work with any external resistance (140 ohms of resistance prevents it from switching when a battery is directly connected).

    The relay I'm using is this (RP68Y):

    I assume the PN2222's resistance simply doesn't drop by enough, so is it possible to bias it's output in some way or should I look for different relays?
  2. duke37


    Jan 9, 2011
    The relay coil has a resistance of 100R, at 6V this means 60mA.

    The relay in series with 240R will take 26mA (2.6V) on 9V - not enough.

    The specification says 4.5V minimum on the relay, with 0.7V acoss the emitter/base of the transistor, you need 5.2V on the base. If you use a low value potentiometer, then the phototransistor will never be able to pull the base up to this level.

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I'm surprised the load is in series with the emitter. This introduces negative feedback which you probably don't want.

    Try moving the relay to the collector side of the transistor.

    Also note that if the interruption is very brief, the relay may not have time to release and then pull back in again.
  4. sid2286


    Aug 24, 2011
    Try with the load at the collector! it should work!
  5. QwertyTSecond


    Jul 31, 2011
    Right, here's the full circuit I've got at the moment:

    The codes correspond to the Maplin order code.
    Thus far, I've found that the circuit works fine if the relay is replaced with an LED, and the relay works fine on it's own. Just the whole thing doesn't work together.

    With the IR LED shining on the phototransistor, the PT's resistance is ~11K
    With no IR on the PT, it is ~60M.


    Okay after flipping the resistor and phototransistor around it's still not working.

    I did, however, find that by shorting out the phototransistor in the first circuit, and dropping the resistor in the second, the relay turned on.

    I'll do the maths once I'm back to health.
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