Problem with Electromechanical relay?

Draw a schematic of your setup, then photograph it or scan it and post the image file to your Flickr account here on Yahoo. Put the link to the image in your question.

It sounds like your setup isn't wired correctly.

What output from the 8870 are you using ?

You have not described the relay you are trying to operate.

What is the voltage rating of the coil ?

What is the resistance of the coil (measured with a meter) ?

The 8870 integrated circuit cannot drive inductive loads like a relay, you might have damaged the circuit within the chip, and now it can't drive a transistor either.

See 'relay driver' in the link below. The microprocessor output can only supply 1mA or something. The relay may need 100mA. A 2N2222 transistor can supply the current easily, and also allows a different supply voltage for the relay. The resistor between the digital i/o (data) and the transistor base defines base current, and stops that from pulling the voltage down too.

It is also possible the relay draws enough current to pull the 5V supply down? Measure coil resistance, check current with 5V using ohms law etc. Also the diode or transistor may be wrong way round, or you have some faulty component.

I'm not sure of the specifications of your individual devices but I've seen many times that people don't realize the relay coil has specific needs.

One thing is that it's an inductive load; like a motor does, for just an instant it acts as a plain wire (before the magnetic field builds), which means a moment of heavy current draw.

You will almost always find at least two transistor control interface to prevent this loading effect. In the case of motors they use four to allow for two for each direction of spin (called an H-bridge).

I think for this case a diode to suspression surge voltage on emr is needed.