Front panel design question

[ttlworks]

Another option would be building a hexadecimal to 7segment decoder with 2*74138
and some diodes, but I'm not sure if this would be fun.

BTW: decoding which segments have to be turned off certainly will require less diodes
than decoding which segments have to be turned on.

[DerTrueForce]

This design is less complex, and would probably work better. It also uses less ICs. Trouble is, this one looks like it will work for a common-anode display, but (if I'm right) it won't work for a common-cathode one, which is what my DVD player pull is. I have two individual displays that are common-anode, but I'd need four more digits to do this, and Jaycar doesn't seem to carry four-digit, 7-segment LED displays(apart from an arduino one). They also don't carry any hex-to-7-segment decoder/drivers.

Of course, if I really want a low part count, I can always use some sort of programmable logic, but then I'd need a programmer for that.

[ttlworks]

There are displays with an integrated latch plus decoder, like the HDSP0962,
but they tend to cost an arm and a leg:

http://www.mouser.de/ProductDetail/Broa ... 52b4SRY%3d

[ttlworks]

Sorry, submitted my last PM while you were posting.

Don't have any experience with the 7228,
but when looking at the 7228 datasheet:

ICM7228A, ICM7228C are for driving common anode displays,
segment driver output impedance is 75 Ohm.
ICM7228B is for driving common cathode displays,
segment driver output impedance is 100 Ohm.
Edit: the resistor values above might vary for different manufacturers.

...it's an interesting question, if you should place resistors
between the segment driver outputs and the display.


For ICM7228A, ICM7228B, a control word has to be written
to configurate display mode and to start a sequential
data write.

For ICM7228C, hexadecimal decoding can be selected
by pulling pin 9 HIGH, no control word,
but the chip doesn't support sequencial data writes.


Those three chips have a different pinout.
Take care.

http://www.mouser.de/Search/Refine.aspx?Keyword=icm7228
BTW: also take a look at the ICM7218.

[ttlworks]

From your schematic, it looks like you are out to tap into the digit driver outputs
of the 7228 for generating the 3 Bit address for multiplexer control and
7228 register write.

I think that using a counter (74163) for generating those 3 Bits would be
less trouble:

1) You don't know for sure yet, if your display has common anode or common cathode,
in other words: if the digit driver outputs are active high or active low.

2) If all the segments of a digit are on, it is questionable that the
digit driver output would have a proper TTL logic level voltage.

3) From the datasheet, the 7228 turns off all the digit driver outputs
for about 10us while preparing to display the next digit.

4) When using a 7228 which requires writing a control word for setting
the display mode, a 74163 could be wired up to count from binary
0111 to binary 1111, for writing a control word and 8 data words
to the 7228.
The uppermost Bit of the counter output then could be used for switching
between writing a control word and writing data.

[DerTrueForce]

The trouble is that it all depends on what I want to use, and how much I'm prepared to spend. If I'm to use the display I have currently, I need a common cathode driver and circuit. OTOH, if I want to use the two discrete displays that I have, I need a common anode driver and circuit, and I also need 4 more digits of common-anode display.

I was trying to decide which was better, but now I'm thinking that it may be best to simply avoid the question, as this business of trying to select 4 bits at a time is being a big pain. I think I'll go with my original display idea, which was use three DIL bargraph LED displays driven by the buffers through resistors, and just read the binary out of that.
The discrete LEDs I currently have appear to be the same brightness whether I have a 1K, a 10K, or a 200K resistor in series with them*, and if the bargraphs work the same way, I'll be very well set for visibility. That will make the wiring much more linear.

Even if I do go with a binary display, I think the hex switches are a good idea. Less components means less wiring, and less possibility of IC-melting error. I really don't want to break any of the ICs I bought from Mouser, as I don't have any spares.
So, could anyone recommend a good hex switch?

* 5mA, 0.5mA, or 25 uA at 5v, if my calculations are correct

[DerTrueForce]

Bump! in the night(sorry).
What's a good life for a hex switch? 10,000 operations?