Power box
Dew Heater enhancement control enhancement
Contents
As this is quite a large project, I've decided to split this project into several pages. Each page dealing with a different aspect of the project.
Overview
From experience using my dew heater, I found that it wasn't easy to tell what setting the heater is on in the dark. To make this extremely easy, I designed (with a little help) a circuit that will visually display the setting of the heater. This circuit will attach in between K8004 and the 1K pot from the original dew heater. It will display the setting in the same style as a VU sound level meter.
Circuit diagram
Here's the circuit diagram for the bar graph.
Here's how the circuit works... The POT VR1 is the 1k Pot taken from the original dew heater circuit. The three connections vRef, DCIn and Gnd, are the three connections that the Pot was originally connected to. This is the voltage that we'll be measuring.
SW2 us used to switch the chip from DOT mode, to BAR mode. In DOT mode only a single LED is lit, this means that the enire ciruit will used about 10mA of current. In Bar mode, with the heater at 100%, the circuit will pull about 120mA of current. When building the final circuit, this switch will be a pair of jump pins, like the ones used on computer mother boards, and hard disk drive.
As I'm want DOT mode to be available, I needed to add the resister R5. The reason for this resister is that there's a quirk on the chip. When operating in DOT mode, of any of the LED's are lit, pin 1 of the chip will pull about 500uA, this is enough for the diode to light about about 1/4 of the normal strength. Placing this resister across D1 will mean that some of the 500uA is drawn away from the LED, stopping it from lighting. When the LED is supposed to come on, it will be slightly dimmer than the others, but it's not noticable, even in bar mode.
I then made this into a PCB, and added the circuits. Here's a link to the board layout image PCB Layout. When you print this it make sure that the long edge of the boarder is exactly 100mm long and the short edge is 75mm. This is the same size and the size 1 boards from Maplins. For my boards I used the SRBP board, as it's cheap.
Although I've finished making up the PCB and tested it. I've not quite finished. This circuit won't be complete until I've worked out how, and fitted it into the box.
Note: If building this part of the project again. I would spend some time re-arranging the positions of the components and tracks so that I can drill a large hole. This hole will allow me to insert a screwdriver, through to the battery monitor circuit, so that I can trim the potentiomiter easily.
©Copyright 2005 Colin Dawson
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