I bought this multimeter(Minipa ET-870C) a while ago for $17, great value. I got it because its nice to have around multiple meters for when you wanna measure both input and output voltage/current. I believe it was advertised to have an auto-off feature for 15 mins but it didn’t. This eventually lead to many drained batteries because I often forgot to turn it off after using it. So during a boring weekend when the weather outside was bad I decided to add this nice feature to the meter. I knew it had to be a small circuit to be able to fit inside the multimeter so I picked the tiny25 the smallest micro I had around.
I quickly put together a simple schematic in Eagle, just the mcu, a voltage regulator a npn transistor and the associated capacitors and resistors. The circuit is powered from the multimeter 9V battery and cuts off the ground path to the meter to turn off its power. No ISP connector was placed on the board to save space but extra long pads were added so I can solder some wires for programming the mcu.
The npn transistor has its base pulled down so when there is no signal coming from the mcu the transistor is off. The pcb was made using the photo etching technique and it was assembled with some solder paste and hot air gun. I also soldered some wires to connect to my MKII programmer and I started writing the code.
To keep track of the time I used Timer0 to generate an overflow interrupt. Knowing the CPU frequency and the prescaler we can find out how often that interrupt will occur. Knowing how often the interrupt will occur we know how many times we need that interrupt to trigger to account for a given time in seconds. This is all calculated inside main.h where you can also specify the time in seconds.
To start the counting process I’m using a tact switch connected on INT0. When the button is pressed the level changed interrupt will trigger on INT0 and we change some flags to start counting as well as turning on the transistor to turn on the multimeter. When the counting reaches the setpoint, the flags reset and the transistor is turned off, thus turning off the meter. At this step the code was running ok except that for a 15 mins period there is ~12s error. I suspect this is because I’m using the internal oscillator which is not very precise.
Since the circuit was intended for extending the battery life, itself had to consume as little as possible. All measurements were made right before the voltage regulator at the battery leads. With no optimization the current draw at 9V was 3.67mA. The tiny25 has 3 sleep modes: Idle, ADC Noise Reduction and Power-down mode. The Idle mode would be good for when we are counting until the setpoint is reached, and the power-down mode for when we are waiting for the button to be pressed(INT0 interrupt trigger). So I’ve added some new lines of code to set the processor into Idle mode when the button is pressed, and to set it to power-down mode the the setpoint is reached and the transistor is turned off.
// Prepare for sleep mode in Idle mode so Timer0 can continue working and providing interrupts
set_sleep_mode(SLEEP_MODE_PWR_DOWN); // Prepare for sleep mode in Power Down mode
With these new settings the numbers are: in Idle mode the current draw is 3.57mA(only 0.1mA saved) but in power-down mode the current draw is only 17.9 uA(a whopping 3.652mA saved). This will ensure the battery life is not affected by the small timer circuit.
Now that everything was finished all I had to do is mount the small pcb inside the meter. Luckily there was plenty of space between the LCD and the logic board. I attached the pcb with some double sided sticky tape and drilled a hole for the tact switch in a pre marked place(this was probably the place designated for an original on/off-standby button that would of reset the internal auto-turn off feature). The tact switch was fixed in place with some hot glue. The wires were carefully routed on the side of the case and everything was assembled back together.
Now I know what you’re thinking, this mod makes the meter more dangerous with all the wires running so close to the high voltage input jacks but this meter did not have much protection to start with. Only basic 250mA fuse on the small current scale so I don’t think I’ve made it more dangerous than it already was. I really don’t recommend getting this meter if you’re working with high voltages or currents. This is only suitable for small currents/voltages where the input protection isn’t going to make a big difference.
Also you should know that the mod will certainly void your warranty if there is one. I certainly didn’t had any real warranty with this meter.
The source code and Eagle schematics are accessible on github here. As you can see this was also a nice opportunity to get me started with using a code repository. And here are some pictures from the construction/assembly process: