Hey Guys!

Im working on a rather open ended project that I could use a bit of input on. I am to design a timer circuit to trigger another device such as a FET. Its only requirements at this point is that it counts down from a range of 30 min up to a full 24 hours, powered by a small commercial battery, and have some sort of feature to ensure it doesn't begin counting accidentally . Not sure really how I want to approach this. Should I be looking more towards using a micro-controller such as the PIC16F1828 or using logic gates, silicon oscillator, etc? Id like to keep it low power and as compact as possible using surface mount components. I will eventually take this to a PCB layout using PADS. Any input/ideas to help me get started with the design process or function blocks would be greatly appreciated!

 

Definitely an ultra low power MCU is the way to go.
How much experience do you have designing and programming with MCU?

 

I have chosen to go the microcontroller route with an internal oscillator. I don't have a whole lot of experience in the microcontroller realm (with the exception of a small projects using an Arduino). I plan to power the microcontroller off a 20mm coin battery that will meet the power specs of the microcontrollers I have narrowed down to. I will also likely implement a push button switch for a 'start' function and a 2 directional see-saw style switch for increasing/decreasing time.

 

How do you plan on selecting and displaying the count-down time?

 

I would like to keep size relatively small so I have pretty much eliminated the idea of using 7 segment displays. I was thinking of possibly using a small, low power LED alphanumeric display. But, I am worried that it will draw too much current from a small coin cell battery, should I use that as a power source.

 

LED will definitely draw too much current while the basic MCU only needs less than 1μA.

I think you have to do some brain storming to figure this one through before you go any further.

 

how about a flashing led? push a button and number of flashes is determined by status? also, there are several ham radio porjects that read out frequency and such by morse code.

 

I did think of the flashing LED option as well and that may be a good route to take. I'm also concerned about how many times this device could run at its upper limit of 24 hours before the power supply would shut down. Even if I used a buck/boost between the coin cell and the microcontroller..

 

At 1μA the timer can run for 10 years on a 3V coin battery.

 

But if I end up attaching some display for user interface or adding external circuitry to the microcontroller outputs, this would likely overload the current capability of the battery and cause the supply to shut down much sooner, right?

 

So I would just need to come up with exact specifications, in terms of current consumption, from the added circuitry and divide the battery capacity by that value to determine its lifetime at continuous use?