So, for a project I'm using a light sensor to switch on a relay which drives a motor, as seen in the pic below

circuit diagram

Q1 is 2N2222A NPN transistor, Q2 is an NPN phototransistor, R1 is a 100k pot

The relay is a 0.5A 5VDC SPST DIP PCB Mount Reed Relay (http://www.altronics.com.au/p/s4100a-0.5a-5vdc-spst-dip-pcb-mount-reed-relay/)

The relay is switching a circuit comprised of a 9V battery, 9V motor and an on/off switch.

Currently, when the phototransistor is not receiving light, the relay has about 0.2V across the coil, while shining light onto the photostransistor brings the voltage across the coil to around 4.9V, which closes the switch and starts the motor. However, when switching off the light source, despite the voltage across the coil dropping back to about 0.2V, the switch stays closed.

After looking around, I assume it's caused by arcing between the contacts, as per Relay "sticking". What can I do to prevent this?


  • 1
    \$\begingroup\$ Seems like there were some good suggestions in the question to linked to. It seems your question has already been answered there. \$\endgroup\$
    – Dan Laks
    Jun 10, 2015 at 5:50
  • \$\begingroup\$ Fair point..I guess because I'm very new to this, I wasn't sure how to apply that to my problem. For example, the suggestion to use a flyback diode on the circuit being switched; where would I include it? Before or after the motor? \$\endgroup\$
    – Blaze
    Jun 10, 2015 at 6:22
  • \$\begingroup\$ The sticking contact won't be fixed by a flyback diode, but nonetheless, you should include one on your relay. The diode goes across the relay coil with the cathode pointing towards the voltage source. Google "relay flyback diode". The same search on this website show others who have asked about flyback diodes on inductive loads as well. \$\endgroup\$
    – Dan Laks
    Jun 10, 2015 at 6:30

1 Answer 1

  1. Insert an 18Ω resistor in series with the motor. This will limit its starting current to 0.5A or less.

  2. Put a diode across the motor (diode Cathode to motor +), to soak up any voltage spikes that may occur when it is switched off.

  • \$\begingroup\$ an "18Ω resistor in series with the motor" won't just limit the starting current, it will limit it's max current to .5 amps (and also need to be quite a hefty resistor, 5W worth or so) \$\endgroup\$ Jun 10, 2015 at 12:29
  • \$\begingroup\$ Unlikely to need a 5 W resistor. This is being driven from a 9V battery, after all. \$\endgroup\$ Jun 10, 2015 at 15:57
  • \$\begingroup\$ Putting in the resistor and diode didn't stop the arcing :( \$\endgroup\$
    – Blaze
    Jun 12, 2015 at 6:32

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