1802 Membership Card, Clock build Aug 2018

Last updated Dec 5 2020. Edited by Herb Johnson, (c) Herb Johnson, except for content written by others.


This is one of a series of Tech Notes to support the COSMAC 1802 Membership Card. Follow that Web link for current status of that product and links to the current support and sales Web pages. Here's a direct link to the clock product on Lee Hart's Web page.

[1802 MC clock]

Lee Hart provides an alarm-clock kit as another front-panel for the 1802 Membership Card. It's an LED display hand-built from LEDs to display time in hours and minutes, like other digital clocks. A speaker provides the alarm and the 60Hz power line provides both power and a time signal. On this page, we build and test the clock kit panel. I obtained the board and used my own parts; Lee Hart also sells the board with all the needed parts.

On Lee Hart's 1802 MC page for the clock, there's a current copy of the manual and schematic, and a current version of the ROM source code program and hex codes for the EPROM to run it. - Herb Johnson

Mods to the PC board

[1802 MC clock] [1802 MC clock]

The PC board I obtained from 2018, needed two modifications. The PC trace cut shown, is an inadvertant connection to ground which must be cut out. The wire shown, makes the proper connection to DC power. It's possible that later versions of this card don't need this modification. Check the manual and board accordingly.

Build and test

LED notes

I constructed the clock board a little bit at a time, testing as I go. That's particularly useful in determining the orientations of the LED's. If they are reversed, they just won't light. Different models and types of LEDs have some mechanical orientation - shape of the LED, length of the wires - to indicate the anode (positive end) of the LED. But these vary. So practically, figure out the orientation of YOUR LEDs and go with that.

[1802 MC clock]

In this photo, I've built enough of the circuit to produce the 60H signal from the 9 volt AC wall wart recommended. The clock needs an AC power source, to get the 60Hz power line signal as a timing source. For testing, I've put a 5 volt DC source on the 30-pin connector, using the red probes. In use the clock board gets power from the 1802 MC CPU board on the 30-pin bus.

Again in this photo, I show that I've shorted 74LS145 pin 2 to pin 8 (ground), to ground one end of one of the LED strings. The LED's operate in series pairs - in the photo you see LED1 and LED2. THe other end of each pair is connected to +5 volts. So in this test, I show that I've placed the LEDs correctly, because they light up. NOw I know the correct orientation for these LED's.

Look closely at the empty LED positions, and you'll see a diode signal. That's one means to establish which way the LEDs should be assembled. The board is very consistently designed so that all the LEDs are oriented in the same way, if they are horizontal or vertically aligned (as you'd read the clock in use). Of course look at the PC board information ("silk screen" as it's called) for the orientation of all other components, as described in the manual.

About LED colors. Red and yelow and green, have about the same voltage drop. But blue and white, probably not - they get too bright at 2.5 volts or so and would outshine the red and yellow's. So to use those you'd need a series resistor.

more construction

[1802 MC clock]

In this photo, I've placed and soldered all the non-LED components on the board. I'm ready to add the remaining LEDs.

[1802 MC clock]

In this photo I insert two columns of LED's. If you look closely, they all have little anvils inside where the LED chip sits. And they all look the same - the same as the first two I tested. There's another pair of LEDs which sit at right angles. To get their orientation correct, I follow their connection to +5 volts, oriented the same way the vertical LED's are connected to +5 volts. I confirm that with the silk-screen diode image.

[1802 MC clock]

So in this photo, all the LEDs are in place and the board is complete, except for the 30 pin female header which plugs onto the 1802 Membership Card CPU board header.

All LED's worked under test, which was satisfying. I was puzzled by the behavior of the "colon", until I realized the default inputs of the '145 was "7" which pulled down the "colon". When I pulled up the other end the colon immediately lit and stayed lit during test. The colon blinks every second, just like a desktop clock.

I tested the audio "amp" by simply connecting the input to the "8 volt" AC line. It worked, it hummed. and of course, a voltmeter tests the DC supply variation. Mine happens to max out at 5.5 volts, the LED's go out when it's just a few volts.

Near completion

[1802 MC clock]

And in this photo, I show the AC supply connected through a series diode (white wire pair). I struggled to find a "8V AC transformer" and could not. So I just used a 9v with a series-pair-opposing diodes, to force some voltage drop below 10V. That matters because the voltage ratings of the capacitors are 12V. If they were rated larger (says Lee Hart) they would have been bigger and take too much space. Of course, the open-load voltage was 10V before the diode drop. I looked at a few 9V AC warts to find that as the lowest-voltage. There's an old computer speaker connected with a black/white pairs of wires. Note again, I've used different colors of LEDs to display the different modes of the clock.

I'd like to connect this board, between the toggle front-panel and the CPU, for further testing. So I've delayed added the 30-pin connector. Of course the CPU will need the clock ROM to operate, or I'd have to load the clock program into low RAM.

Herb Johnson

This page and edited content is copyright Herb Johnson (c) 2020. Contact Herb at www.retrotechnology.com, an email address is available on that page..