Thank you all for your help. My relay is driven by a 2N3904, whose base is
connected to the PIC through a 4.7K resistor. I did use a diode (cathode to
+) across the relay, and the 0.01uF cap between Vdd and ground. However,
whenever the PIC output high, the serial communication between the PIC and
the DS1620(thermostat chip) become unexpected. I think the noise or the
magnetic field from the coil of relay might:
1. interfere the communication
2. affect to the normal working conditions of the PIC or the DS1620
    Please give me some advises. Thank you.

----- Original Message -----
From: Reginald Neale <neale@SERVTECH.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, April 07, 1999 11:07 AM
Subject: Re: Relay problem


> > Whenever the 5V output from PIC is applied to relay, the microprocessor
> >will run unexpectedly. If I put relay in a separated board, it's OK. I
think
> >the problem is the noise from the inductance of relay. Can anyone tell me
> >how to eliminate this problem? Thank you.
>
>   Hi Peter. I'll try not to avoid the dyslexia that plagued me
>   Sunday when I tried to help someone else with a similar problem.
>   :-(
>
>   If you could find a relay that would operate on 20mA of current,
>   you could theoretically drive it direct from the PIC, but it
>   would still be a bad idea.
>
>   Don't drive the relay directly. Drive the base of a switching
>   transistor thru a series resistor. For a typical low voltage
>   relay, use a 2N2222 and put the relay in the collector circuit.
>   Use a 470 ohm resistor to drive the base. In parallel with
>   the relay coil, place a general-purpose diode with the CATHODE
>   toward the (+) supply voltage.
>
>   It's a good idea to return the (-) side of the relay supply
>   directly to the transistor emitter, which should also be
>   logic ground.
>
>   Make sure that MCLR is asserted high and that your supply will
>   deliver the current your circuit is demanding from it.
>
>   If that doesn't stop the resets, email me directly and I'll
>   try to help you further.
>
>   Reg Neale
>

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