Nobby

As CW2 mentioned, a short could be occuring. Do you have a series resistance on the transistor collector or emitter? Another cause could be your base resistance. 1kOhm is too small. Assuming you don't use an emitter-follower BJT configuration, the base current into your BJT is 3.3V/1kOhm = 3.3mA. This current might seem small but BJTs have a current gain of at least 50 for common BJTs. When your control pin goes high, the BJT will try to draw 165mA from the 5V regulator. When you try to pull too much current from a supply, the voltage of the supply will dip and cause a reset of the Netduino. You should find out what the gain of your transistors are and calculate the base resistance to work well with your relay. For example, you need about 60mA to operate the relay and your BJT has a gain of 100. The base resistance needs to be in the ball-park figure of 55kOhms. Actual(peferred) values you can buy close to that at 56k and 47k. 56k will give you less than 60mA so I'd choose 47k in this case. For saftey, you probably have or should have a series resistor connected to the BJT collector or the emitter. If you drive a BJT hard enough, it's collector-emitter voltage can go down to 0.3V. If you don't have the series resister in there, you can virtually short the 5V rail and cause the Netduino to reset and possibly damage the board. Since your relay runs on 5V, you have to have a small series resistance so that the drop across it when you draw 60mA is small. Secondly, you don't need the anti-parallel diode since it's only for dealing with protection against negative voltages.

Hey crawf, I'm about to head to bed so I'll read this thread tomorrow. Judging by your code, the methodology is fine. I use I2C comms with a GPS unit and i've tried various approaches to see how flexible the API is. The main points you need to know: -When you perform a write operation, .Net will ALWAYS provide the device address in the transaction, you don't have to add this to the byte[](not that you are but just so you know) -When you perform any transaction, if zero actions are returned from Execute(), this means that communication to the device failed completely. This has nothing to do with register addresses -.Net handles all ACK & NACK signalling(comms interlocking). The number of ACKS & NACKS returned by the slave device determines the return value of Execute(). -You can't simply read a single byte in a transaction. The data you read from a target register needs a destination array large enough to fit it. If you don't know the size required, give one sufficiently large enough. The remaining bytes in the array are set to zero(or a value defined in the device documentation).