MAX Solar Engine

Specification:

Construction Details:

I called MAXIM (1-800-998-8800) and requested samples of the MAX8212CPA (8 pin DIP) Microprocessor Voltage Monitor that had been mentioned on the mailing list. I selected the 8212 over the 8211 using the spec. sheet downloaded from MAXIM's website, as the 8211 has a current limited output sink (7 mA) whereas the 8212 has an open drain N-channel output without current limit, and I didn't have an output stage in mind yet.

MAXIM sent me two as a sample:

MAX8212

I chose the 8pin DIP as the easiest to work with on a breadboard. The device is also available as a surface mount component (3mmx3mm) or in a TO-99 can.

I wired up the circuit as shown in the spec. sheet example, and verified the functioning. The beauty of this device is that the trigger level is set merely by using a resistor divider (R1, R2) to produce 1.15V at the desired trigger.

Not recalling any basic electronics theory, I asked the list for an output driver, and found the simplest response from Paul Atkinson (Thanks, Paul!), and breadboarded the following circuit (sorry for the quality):

MAX 8212 Solar Engine

For my testing purposes I used discrete resistors (all I had): 1Mohm and 1.5Mohm. I also disabled the hysteresis circuit by shorting the HYST input to the THRESH. This gives me (theoretically) a trigger at 1.15x(2.5/1M)=2.875V which is a good voltage for the cap/solar panel/motor combination I was using. The measured components were 0.966Mohm and 1.502Mohm for a real trigger of 2.938V. Measuring the circuit in operation showed then to be very accurate.

Following is the voltage/time profile of the circuit in operation (timed by hand, voltage off of DMM):

The performance of this engine in poor light conditions (ambient overhead flourescent room light) is pretty good. This is due to the low current consumption of the MAX8212 chip (5µA typical, 15µA max). My multimeter only reads down to 1mA, so I couldn't verify this. I tried other discrete resistors, an obtained similar accuracy results. The only restriction is that the MAX8212 needs >2.0V V+ to operate properly, so the desired trigger voltage must be above that.

The circuit as shown discharges the cap to about 1.25V. It drops quickly, so an accurate value was hard to determine. I'm trying to borrow a DMM with a min/max capture to determine these values more accurately. I haven't played with the circuit enough to determine if this shutoff level is determined by the output driver used, or the MAX8212 characteristics.

Lessons Learned:

• Setting the trigger level is simplified if the R1/R2 voltage divider is replaced with a 5-10M precision pot. This can only be done if the hysteresis feature of the chip isn't required.
• This device seems like it could replace the 1381 voltage trigger in most applications (like the photopopper circuit) The benefit is the ability to vary the trigger voltage very simply, with the drawback being package size and the requirement for external resistors making the circuit physically larger. I am currently experimenting moving this circuit to the real world and using a 14 pin DIP socket as the chasis (allowing me to seat both the MAX8212 and the two output transistors in the socket).

(Originally Written May 1998)