Nuts and Volts - November 2014

through a complete system setup.

When it encounters the check for why we started up, the reason will be RESET_INSTRUCTION, and processing will continue to turn on the LED and then into the infinite loop that is the basis for most embedded systems. Within that loop, we check for a button press of two seconds or longer which initiates the HWSleep() function all over again.

The additional hardware on the prototyping board represents control of power to a load (in this case, a buzzer) via a pFET (P-channel MOSFET). Support for this is included in the code file (LF18Switch.c) at the article link.

Applications

Using a microprocessor as a power switch may seem like over-kill, however (as I mentioned earlier), since my projects are usually processor-based, I get the functionality for free. Even the pin for the button is free because I can use it for other operations after the system is turned on. While some systems — such as our LED light — need a switch to turn off, others will turn themselves off automatically after they have completed their tasks. We’ll see that in my wireless doorbell and motion sensor transmitters next.

Before we get to that, there are some other tests I For driving relatively low power (200 mA or less) loads, I prefer to use a pFET. They’re cheaper than a relay, use very little space, and can have a very low voltage drop. Driving a 20 mA radio transceiver, I experienced a “diode forward voltage drop” of only 0.15V. Ordinary transistors will usually have a drop of 0.7V or more. The 2N2222 I tested had a 0.9V drop with the same 20 mA load I tested with the pFET.

For my application, I need to get the highest possible voltage from the battery to my radio. The circuit for this is included in Figures 2 and 3. Note that the pFET is active low. You need a 1M pullup resistor (R5) to ensure it turns off because the output from the processor will float when it’s asleep.

The reason I’m focused on low sleep currents and pushbutton activation is a wireless doorbell and alarm project I’m working on. I want to be able to use a pushbutton switch or a PIR (passive infrared) sensor to trigger the radio transceiver that communicates with the base station. The radio takes a while to power up, so if you just connect the pushbutton between the battery and radio