Relay issue

Discussion in 'The Projects Forum' started by allenpitts, Mar 16, 2015.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello Forum,

    Using this circuit to turn on an LED animation.
    [​IMG]

    It was working yesterday. When I got home from work tonight
    I started moving the PCBs into the implementation and it quit
    working so noticed that the relay was not clicking even though
    the LED was coming on. So I took the relay out to put in a new
    one. When I got the relay out I tested it by touching the battery
    terminals to the relay poles. The relay clicked and closed. So I figured
    it was a cold solder. And put the relay back in. Stopped working
    again. So I tried a new relay. Same result. So for some reason
    the relay works when it is not in the circuit but does not work when
    it has been solder into the PCB.
    Any ideas?

    Thanks

    Allen in Dallas

    PS This is a pictorial the PCB

    [​IMG]
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    Did you check the battery voltage when the relay is supposed to be energized?
    9V batteries have a low capacity for driving a relatively high current load such as a relay coil.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Your relay contacts are shorting out the Base /Emitter of the transistor.
     
  4. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Certainly looks like that. But you say that the relay closes and does not open.
    What output voltage do you get out of the PIR module when nothing is detected? If it does not go below about 0.6V then the transistor may not switch off.
    Try adding a resistor between the base and emitter. Another 470ohm should do. In fact, I would also try raising the value of the resistor from the PIR output to the base. A few thousand (k) ohm would be better. Try 2k2. Depends on the voltage out of the PIR module when something is detected.
     
  5. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello Forum,

    Sorry if I was unclear.
    The normally open pole does not close when the PIR biases the transistor when the relay is in the circuit
    But when the relay is not in the circuit applying 9 volts to the relay closes the normally open poles.

    Maybe I don't understand transistors. I thought a transistor is a just a switch and current only flowed from
    the emitter to the collector. Is there current between the emitter and the base?

    Allen in Dallas
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    Yes. A bipolar transistor requires a base current of at least 1/10 of the collector current to operate properly as a switch.
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    if you look at the points where the base and emitter are,they are connected to the relay contacts,which are shorting out the transistor, cut the track leading to the relay contacts and it should work.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Used as a switch, the base-emitter current should be about 10% of the load current, collector-emitter.

    Gaaa, typed too slow.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,800
    1,103
    Your relay coil is in series with the LED.
     
  10. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello Dodgy,
    Will try that when I get home tonight.
    But just to be clear. In the drawing marked 'PCB Relay w PIR' above
    Is the emitter the pin coming from the PN2222 that is above the middle pin transistor?
    or
    Is the emitter the pin coming from the PN2222 that is below the middle pin transistor?

    Thanks

    Allen in Dallas
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    below to the batt neg.

    Also as Alec-t said, the led is in series with the relay coil, and should be in parallel via the resistor.
     
  12. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello AAC forum,

    Thanks to Dodgy Dave, Alec T, Reload Ron, JDT Crutchow, et al. for your help.
    I finally got it to work. Based on the input from Dodgy Dave and Alec T
    the PCB circuit was reworked:
    [​IMG]

    The PCB and the Velleman MK152 then went into a wheel.



    About half way thru the clip the already unsteady camera work gets
    even jumpier because I wave my hand to retrigger the PIR.

    Thanks again.

    Allen in Dallas
     
    Pencil, Bernard and Dodgydave like this.
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