# Diode Steering And Counting With A 555

While you’re not likely to see this technique used very much today, there’s a lot you can do with a 555, some logic chips, and a handful of diodes. [Fran] is here with a great example of using these simple parts to build a circuit that counts to zero, using parts you can probably find under your workbench.

[Fran] was inspired to build this diode counter from one of [Dave]’s Mailbags and [Colin Mitchell]’s 555 circuit book. The 555 is the standard component found in every parts drawer, but since we have tiny microcontrollers that cost the same as a 555, we’re not seeing the artistry of a simple timer chip and a few logic chips much these days.

This circuit began with a 555 attached to a 4017B decade counter. Simply by tying a few LEDs to the output of the 4017, [Fran] made a bunch of LEDs light up in sequence. Cool, but nothing unexpected. The real trick uses a few diodes and six LEDs to build a scanner – a line of LEDs that will blink from left to right, then right to left. Impressive, and with a little more circuitry it’s a Larson Scanner, as seen in Battlestar Galactica and Knight Rider.

The real trick for this technique comes when [Fran] pulls out a piece of protoboard, several dozen diodes, and seven old transistors to have a seven-segment display count from zero to nine. The 4017 simply counts out on ten pins, and each of these pins is wired to a bunch of diodes for each segment in the display. Add in a few resistors and a transistor, and [Fran] replicated what’s inside a seven-segment driver with discrete parts.

If counting to zero isn’t enough proof that you can do a whole lot with some diodes and logic chips, how about programming an Atari 2600 with one?

Video below.

## 27 thoughts on “Diode Steering And Counting With A 555”

1. superUnknown says:

I’m surprised at Fran’s electron vs. conventional ramble. Why confuse things? Put your head in neutral and take comfort in the warm bliss of conventional current moving from positive to ground.

1. mlagana says:

howabout holes moving from + to – and electrons moving from – to +,ying and the yang

1. Pat says:

Well, it’s not like the electrons are booking it either. Snails practically move faster.

2. Dr. Bob says:

They do in DC.

1. Dr. Bob says:

I forgot the site I’m on. DC as in District of Columbia.

2. Brian says:

No, holes don’t actually move. They appear to move as the electrons are moving and changing the net charge as they fill or deplete shells. This is a common misconception and the argument has persisted since the first P-N junction was created.

1. mlagana says:

Thank you Brian for a constructive reply, hackaday should be about education, not about who can be the most sarcastic when faced with a misconception. I was actually taught or mis-interpreted in electrical eng that holes move. Posting it has cleared up that it is not necessarily the case, thanks to Brian.

2. Alan Kilian says:

But then Vss (Source) and Vdd (Drain) will be wrong. ;-)

But… The P-mosfet’s source is the higher voltage! They should both be Vss!

3. Ok, it’s been a long time since my electronics lessons — are the diodes really doing anything other than preventing backcurrent into the 4017? It seems that the LEDs could be driven directly from the 4017’s pins for the back-and-forth scanner without any change. I’m not getting this “steering” concept — it’s behaving as a wire.

1. Foobar Bazbot says:

Without the diodes, current can backflow through the whole matrix, and all the segments would be lit up all the time.

2. Alan Kilian says:

Take a look at the schematic at 5:38 and imagine removing the diodes.
you would then have two outputs of the 4017 connected directly together which isn’t going to be good when one is high and the other is low.
The diodes allow two outputs of the 4017 to be connected to one LED and if either one of them is low, the LED lights up (and the 4017 doesn’t blow up!)

4. 666blah666 says:

“Counting to zero” sounds very strange if we are talking about an up counter. Why didn’t you say “counting to nine”?

5. chango says:

I thought 10k resistors were “the standard component” in every parts drawer? 100nF X7Rs should come in a close second.

One thing I’ll miss about Radio Shack is the 50-pack of 1N914s. \$3.50 and available at the corner for impulsive mad science like this. Those and the pack of 20 random LEDs.

1. Ty Tower says:

Fran and I have had differences before but I have to agree with her on counting 1 to 10 not zero to 9 as after all zero is an absence of anything not a number. The holes moving theory was invented by a teacher who wanted to hide the fact that current flow teaching was wrong . Fran is right and the teachers should show similar common sence and start teaching it right . Electrons flow from negative to positive and the arrows need changing . Simple as . Once thats taught the wrong will disappear quickly. A bit lengthy that one Fran and some repetition but still enjoy your work.

1. Except for hole flow is an incredibly useful model when you get into semiconductor analysis. At the scale we’re looking at it doesn’t really matter what is moving where. The most important thing is to simplify the math involved that describes the system.

1. Ty Tower says:

I guess you were one that learned it wrong and now can’t shake it. In such a field seeing it how it is would be the most necessary part. With conventional current flow you have one hand tied behind your back

1. Ty Tower says:

I meant to add . The hole theory again is an attempt to give a name to something that is an absence of anything . . Innitially in a battery for instance an anode starts of with a given manufactured charg (assumed here) and then under charging electrons flow from the positive onto the negative plates . So that leaves the anode with no electrons or a void that can be filled by electrons when the plates are connected . Capacitors are the same and all the electrons are at equilibrium at the discharged state and flow to the negative plate on charge which leaves a void on the anode plate .
It is the void that the old teachers called holes but like zero as a number it just ain’t right.

6. patrick says:

Very good article. Not all of us grab a up for every design or project. This will go into my folder for future designs.

7. This is essentially how diode ROM works. Used to be common in small PBX systems and sorta-common for defining constants or bootstrap loaders in the Bad old Days when burning an EPROM was a chore.

8. DimaP says:

Nice project.
The diode steering is really good idea.

9. richfiles says:

Well, here’s my diode steering circuit. This has been planned for 6 months… and I built it in one night…
Not really sure how. LOL

http://i.imgur.com/L4yUvy0.jpg

If you treat it as a diode ROM, it’s basically a 270 bit ROM with 100 ones and 170 zeroes. The purpose is to encode “unit characters” for a display that will interface to the video game “Kerbal Space Program”. It is capable of displaying m, km, Mm, Gm, Tm, Pm, Em, Zm, Ym, m.S, ΔV, –, as well as an all segments test character. It consists of two separate modules to drive a pair of 14 segment alphanumeric displays. It has 30 outputs, 13 inputs, and has an additional 8 diodes to trigger the second display when certain characters on the first display are selected (eg. selecting “m” on the second display when displaying an SI unit prefix in the first display).

http://i.imgur.com/f2zayIT.jpg

The photos don’t show the 8 additional diodes or the interconnection cable that plugs into the SIP sockets on the two modules to link them together.

1. richfiles says:

BTW, Second display is not lit due to diode fan-out… I did not have sufficient quantities of the resistor value on hand required for the segment LEDs. Instead, i set my bench supply to current limit at the forward bias voltage, which certainly protects the LEDs, but the voltage drop for the second set of diodes int he second display is too low for them to light.

The second module has been tested directly, and works. Both will work together once all the LEDs have proper current limiting resistors soldered onto them and I can run the circuit at the full 5 volts it’s intended to operate at. Forgot to add this detail to the original comment. Oops!

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