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basically I am follow the Schematic in this link- https://stackoverflow.com/questions/14833999/arduino-switch-control , sch

I can get the transistor to switch on/off a motor, but now instead of a motor, I try to switch on/off a window alarm similar to this one https://www.amazon.com/Window-Door-Alarm-Kit-Pack/dp/B007RUGANC alarm, but the problem is the alarm is already on even there is no supplied base current, please advise how to resolve this.

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    \$\begingroup\$ Are you sure you have the diode's polarity correct? \$\endgroup\$
    – HandyHowie
    Commented Aug 15, 2017 at 12:25
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    \$\begingroup\$ Such an alarm is a completely different kind of load compared to a simple motor. The alarm has electronics in it and a battery. It is not trivial, especially for a beginner, to modify the alarm so that you can switch it on and off like the motor. You'd have to reverse engineer the alarm to get a schematic and then it might be possible to find a way to switch it on/off remotely. \$\endgroup\$ Commented Aug 15, 2017 at 12:26
  • \$\begingroup\$ That alarm has no external terminals, so I have no idea how you connected it. \$\endgroup\$ Commented Aug 15, 2017 at 14:30

2 Answers 2

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At a guess, your problem is the use of a TIP120, combined with the peculiarities of the alarm.

First, since the alarms are battery operated, I'd expect them to operate on very low current. This is possible with piezoelectric buzzers.

Second, TIP120s are designed to drive high-current loads with very little base drive, but they are not optimized for low OFF current (also called collector cutoff current), and are rated a maximum cutoff current of 0.5 mA with no base drive. I suspect that this is your problem. When off, they still allow enough current to pass to operate the buzzer.

There are two approaches you can take. The first is to recognize that, with low operating current, you don't need a high-gain Darlington device to drive your alarms. Try replacing the TIP120 with a 2N2222 or 2N3904. Alternatively, use your TIP to drive a relay of the appropriate coil voltage, and use the relay's contacts to drive the alarms.

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  • \$\begingroup\$ thanks, you are probably right. before I asked, I tried with other transistor with no luck, and I also prepared for the worst scenario to switch a relay. \$\endgroup\$
    – andy
    Commented Aug 15, 2017 at 13:20
  • \$\begingroup\$ can you suggest a relay package? the smallest package relay if possible, Electromechanical Relays is too bulky \$\endgroup\$
    – andy
    Commented Aug 16, 2017 at 9:36
  • \$\begingroup\$ I am a mechanical guy try to do some electronic design for my own, that is frustrating :) so can you suggest a relay package? the smallest package relay if possible, Electromechanical Relays is too bulky. \$\endgroup\$
    – andy
    Commented Aug 16, 2017 at 11:47
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You can try to add some pull-down resistor on your base line it might be occuring because of the noise.

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  • \$\begingroup\$ tried that too, no luck \$\endgroup\$
    – andy
    Commented Aug 16, 2017 at 9:35

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