Vague Terrain

In this section I'll refer to pictures of a finished board and a schematic. Accompanying this article is an archive file (conveniently provided as .zip and tarball, as you prefer) that contains:

1. a read_me file describing full contents in detail
2. all files needed to modify or have manufactured MMMSE boards of your own
3. a schematic in .png format
4. custom component and footprint libraries for Kicad PCB design software
5. relevant datasheets
6. tuning tips documentation
7. other stuff

Brief Technical Description: This circuit is based on the MAX8212 voltage supervisor integrated circuit. It is meant for "supervising" the power supply for, say, a computer and give a warning if the voltage is too high or too low. This behaviour can be adopted for use as a solar engine. I learned about this from the manual for a Solarbotics kit and the MAX8212 datasheet (pdfs are in the tarball).

Basically, the chip watches its supply voltage. When the voltage reaches an externally set upper trip point, an output pin goes active low. When the voltage falls below an externally set lower trip point, the output goes open. It does this via an external resistor network (2 trim pots) interacting with a precise internally generated 1.15 volt reference. You can hook-up the active low output to a PNP transistor and drive a motor or something.

My Modifications

I found a few things I didn't like about the original circuit, so I:

1. changed the configuration of the trim pots so upper and lower trip points can be set independently. Previously, adjusting one would affect the other- very annoying.
2. added a "kill" point which, when grounded, instantly shuts off the MAX8212. Handy if you want an external signal from somewhere (eg. limit switch, microcontroller etc.) to turn off your machine instead of waiting for it to die out
3. beefed up the transistor and added all kinds of filtering (clamp diodes, capacitors) so heavier loads and bigger motors can be controlled

MMMSE In Operation

1. Inputs and outputs

The MMMSE has a four pin input and a four pin output. I used what are easily my favourite new green connectors. They can be used as screw-in terminal blocks or pluggable connectors.

The inputs are labeled PV-SCP IN on the top of the board. There are two sets of positive and negative inputs: one set for your charging device (ie. solar panel) and another for your storage device (ie. super capacitor). It doesn't matter which goes where as long as the polarity is correct.

The four output pins are on the left, labeled SOLENG OUT. Each pin has it's own symbol. Only two of them are must-use. The other two are optional:

K : Kill pin. Well, actually this is an input. Grounding this pin will turn off the MAX8212 from an external source. You can leave it disconnected if you don't want to use this.
- : Ground pin. You need to hook this up to ground on your external device.
< : Output pin. This is the transistor output. It is active high and will provide positive voltage to your device when the MAX8212 is on.
+ : Positive volts. Convenient direct access to the positive input voltage connected to PV-SCP in. You don't need to use this if you don't want to.

2. Trigger Voltages

The upper and lower trip point are set by 2 trimmer potentiometers. They are on the bottom of the board and labeled L-trip and H-trip respectively. They can theoretically be set anywhere between about 1.5 and 18 volts. With the 1 Meg pots I used, I found I could, in practice, set the voltage between about 3 and 15 volts. Other trimpot values are possible, check the datasheet or experiment. There's a tuning tips text file in the tarball with more info.

3. Output Transistor Option

The output transistor must be PNP. You can use either a TO-220 package Darlington power transistor if you want to drive heavy loads, like motors. I found I couldn't get it to work with MOSFETS. I used TIP107, others would work (TIP125, TIP127 etc.). There's enough filter caps and diodes to withstand serious spikes. This is overkill if you are just signaling a microcontroller or something, so for that you could use a smaller TO-92 transistor like 2N3906. You could also omit the diodes and filter caps if the load is non-inductive, ie. keep them in if you are driving a relay coil. There are 2 sets holes on the board for either type of transistor in the bottom right corner. Don't use both!

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