What I’m trying to understand is how to make a slightly smarter circuit for the Roof-Link project (the mesh network of Xbee chips that send sensor readings from the 12th floor solar panel, down to our department on the 4th floor). What happens now, is that the 12th floor module senses the voltage coming in from the solar panel, and sends that information out through an xbee radio. This circuit is powered using the amps coming in from the solar panel, so once that drops down under 3.3V, no information is sent. I would like to be able to detect this drop down to 3.3V and have the Xbee send a series of zeros, stating that it is going to be sleeping (so instead of just null, get a few zeros before going null).
Another useful place for something like this would be the 8th floor module, which is currently used to pass data down from the 12th floor to the 4th floor. It is powered by a rechargeable battery pack. The Xbee radio chip is programmed to go into cyclic sleep, enabling maximum energy saving. It would be a good feature, to have the circuit detect the battery voltage going down, and having the xbee send out a signal before it dies, rather than just going silent.
For this reason, I’ve been playing around with the MAXIM 8212 voltage detector chip. Using the given solarbotics circuit. Doing a couple of tests to see which resistors (R2 and R3) I need in order to detect an input voltage level of 3.2-3.3. From the diagram in that pdf file, it seemed like R1=100k, R2 (trigger)=175K and R3 (hysterersis)=275K should do the trick. This triggers the load at a voltage of around 6V, and releases when it goes under 3.3V.
Now if I want the trigger to be around 3.5-3.7 (fresh battery) I need to place R2 = 200k. And to know that the battery’s power is ending (3.0-3.2 V), the hysterersis resistor value has to be R3 = 275k.
The 1381 IC is a voltage trigger. Using this IC in a circuit (I got the 1381S, which has a preset voltage trigger point at 4.0-4.3 V. I just wanted to try to make a simple BEAM circuit here, wakes up whenever it has enough voltage (4V in this case) to power the load. The 2N3904 is ***(goes ). Again here, I got a trigger from between 2.0 to 2.3V, half of what I’d expected.