Is there anything I can use to replace this circuit?

Its a very simple electronic switch circuit, (transistor+zener diode+resistor)

The circuit is connect to a remote control, the remote control is activated via http request via the arduino. which is controlled by an iPhone app.

Remote control, when the circuit is closed (ie two wires are connected), it is activated.

Problem, when I unplug the circuit, or when my power goes down, it will set off the remote control and activate it!

I need the circuit to be normally open, when no current is applied.


Zener diodle IN4007

Transistor 2N2222

Resistor 1k

Note: I dont want to use a relay, not for something this simple!

What do I do? alt text http://img689.imageshack.us/img689/5518/circuitc.png alt text http://img688.imageshack.us/img688/1032/201072415pm.jpg

  • 2
    \$\begingroup\$ you need to describe your circuit. the application sounds like the sort of thing usually accomplished with a resistor and an NPN BJT, which would go open-circuit in the absence of input. So, what type of transistor is it? What's the zener's voltage and how is it connected? \$\endgroup\$ – JustJeff Sep 11 '10 at 13:03
  • \$\begingroup\$ see description above. :) \$\endgroup\$ – JustRob Sep 13 '10 at 3:44
  • \$\begingroup\$ the 1N4007 is not a zener, it's a basic power rectifier type. \$\endgroup\$ – JustJeff Sep 13 '10 at 22:20

It's hard to tell from that picture, but it looks like you have maybe two wires from the arduino going to the breadboard, and two wires going from the breadboard to that thing on the left. Assuming the thing on the left is the remote control, and that, from what you said shorting its two wires together turns it on, then it's very possible to have, as you say, a transistor that's normally open.

The usual thing would be to use a cheap NPN transistor like a 2n2222 or 2n3904. The collector of the NPN would go to the more positive of the two leads from the remote. The emitter would go to the other remote lead. (Use a DVM to be sure which is which, you can't always count on insulation color!) To ensure that the NPN passes no current when no input is connected, you can put a 50K or 100K resistor across the NPN's emitter and base leads. Then, to get input from the arduino, connect the arduino's ground to the NPN's emitter, and connect the arduino control signal you want to use to the NPN's base through a resistor in the range of 1K to 2K ohms.

A circuit like that should not activate the remote when you disconnect the arduino.

  • \$\begingroup\$ I have something similar to what you described here, expect i didn't use a 50k resistor at the emiter/base leads \$\endgroup\$ – JustRob Sep 13 '10 at 3:48

I cannot ascertain much from the photograph and your description. Could you please sketch out your circuit schematic and post that instead?

  • \$\begingroup\$ done :) see description above. \$\endgroup\$ – JustRob Sep 13 '10 at 3:44
  • 1
    \$\begingroup\$ this should have been a comment, not an answer. \$\endgroup\$ – stevenvh Jun 2 '11 at 14:37

First, the 1N4007 is a rectifier diode, not a zener. It's not very clear from your sketch what it leads to (V+?), but it doesn't belong there! Normally you would place a pull-up resistor here, but I guess the remote does have one.

Remove the diode. When you switch the power off it will pull the transistor's collector to ground (minus a 0.7V drop), which indeed may activate the remote.


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