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Re: using a 5V reed relay to activate your door lock

The Axis GPIO pins can't source or sink enough current to drive your door
lock directly. I have used a 5v DC reed relay to switch external loads
similar to your door lock. In the U.S. you can get a simple SPST relay from
Radio Shack  for $2.69 (part no. 275-232). The relay coil looks like a 250
ohm load so it draws just 20mA at 5V. The contacts are rated 1A at 125VAC.

If your 82/83 board has an unused serial port then you can use the DTR or
RTS pin to drive the relay coil and avoid the trouble of running wires from
the parallel port through the enclosure. The RS232 drivers on the 82/83 are
low power chips and can't source 20mA. I have measured 13mA of current
through the coil using the DTR pin and that seems sufficient to keep the
relay closed. RS232 signals on the Axis board swing from -7V to +7v so be
sure to put a diode between the DTR pin and the relay coil. When you drive
the DTR pin low the diode will be reverse biased, no current will flow and
the relay will open. Otherwise, the flow of current through the coil will
simply reverse and the relay will remain closed. The common 1N914 diode
should be sufficient.

Attached is a diagram illustrating a simple circuit that has worked for me.

Ken Klinner

----- Original Message ----- 
From: "Iain McKay" <Iain.McKay@xxxxxxx.com>
To: <dev-etrax@xxxxxxx.com>
Sent: Thursday, August 28, 2003 7:04 AM

> Hello, I wonder if a reader on this mailing list can help me ?
> I'm looking for a security and door-entry solution, for internal use in
> company.I have a query which will help make my decision as to whether to
> for an Axis Developer Board 83 or for another vendor's otherwise-inferior
> 68000 microcontroller variant.
> The question is, how might I attach a relay onto a 82/83 development
> connected to the digital i/o port such that it could switch a magnetic
> lock on/off, which requires 12V, 250mA ? I presume the GPIO port is 5V
> but how much current can it source/sink ?
> I'm assuming this has been done by others over the years, and if anyone
> could point me in the direction of a schematic diagram that would be
> Kind Regards,
> Dr Iain McKay.
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> Graham Technology plc                               http://www.gtnet.com

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