Crawling Light.

Another small Project.

The idea
Sometimes - as in this case - the stock of electronic components available may offer some inspiration for a new project. This one results in a small device, which itself displays crawling light and is also able to control drivers of more powerful lamps or even soundgenerators.
Since the circuit works differently from what is usually considered 'running lights', I have chosen another - rather obvious - name for it.
Master The mastercircuit
... is made around a hex-buffer. A handful of cheap 100 nF ceramic capacitors, various resistors and a single transistor make do for the rest. The construction is suitable for further experiments and add-ons, so for that end some testpins and a diode (to give some protection) are employed too.
The power consumption is rather modest and batteries (6 - 12 V.) as well as any reasonably stable powersource can be used.

<= click for a different view!

... is vastly dependent on the size and shape of the actual components. Here a couple of SIL-resistor-arrays were employed. They may, however, be replaced by small, conventional resistors without changing the basic lay-out significantly.
In case you decide to try the "crawling light", you may get some help from the illustrations. Please observe, that a couple of the connecting wires (not drawn in red!) run on top of the board -- and that my solution does not have to be the best one!

Click for bigger views =>

Bottom view. Top view
The circuit diagram of the master
Please notice the minus-/common-ground-rail runs from the capacitor top, left and down right.
Except for the hex buffer-IC (= 4050) there are no special requirements to the components.

How it works
The very moment power is turned on, the capacitor at the input of gate 'A' will carry no electrical charge and the input is 'low'. Consequently all of the gates 'B' to 'F' are low too and the transistor 'off'. The capacitor at input-A is therefore charged slowly through the 22k and 1M resistors. After some time threshold-level for the A-gate is surpassed and its output switches to 'high', turning on the first Light Emitting Diode (LED) and starting to charge the capacitor at 'B' a.s.o.a.s.f. The last of the gates to go high is the F-gate. This turns the transistor 'on', and starts discharging the capacitor at 'A'. A new wave of 'going lows' will then proceed from 'A' to 'F' resulting in the transistor being turned 'off' again and the cycle repeating itself all over.
The spectator will see the LEDs one by one turn on until they are all lit. Next the first-lit LED will switch off followed by the rest a.s.o.
Driver Add-ons: more light
If the 'crawling light' is to be part of a toy or a display, higher intensities are needed. For that purpose an external driver will be useful. I have tried a simple approach like the one in the diagram. As you will see, I could not stand the temptation to use the first driver as an inverter for a second one. The first trio of LEDs does not, in this configuration, extinguish completely.
The drivers (6 of them) run readily on a separate powersupply and a plain rectifier will suffice. Resistance values are calculated for 12 V. and 3 yellow or green LEDs. In case you are bent on red LEDs, just add an extra one, making it to 4 diodes.
The drivers, when connected, will prevent the LEDs in the mastercircuit from lighting up.

Connect here!
Add-ons: sounds instead of light.
Small, adjustable LF-oscillators are readily made around CMOS-Schmitt-trigger-NAND-gates. These come in packages of 4 (= 4093), so 2 of the kind were required for this purpose.
The value of the capacitor 'C' has to be determined experimentally because the internal parameters of the 4093 affect the frequency a lot. 1 nF is a good place to start. It is not adviceable to go lower than 220 pF.
Another peculiarity was discovered during my experiments: apparently there is a smudging effect amongst gates in the same housing, making two oscillators, running at nearly the same frequency, 'lock' to one another. The problem was partly solved by involving the vacant two NAND-gates as stand-ins for the troublesome ones.
The 'logic' outputs from the mastercircuit do not match the controlling input in the oscillators and some modifications have to be made. You may choose from two possibilities: either remove (or omit) the LEDs or establish an extra connection going right to the bufferoutputs.
The oscillators are best run from the same powersupply as the mastercircuit.
Since the LF-oscillators generate square-waves, the sound does appear rather harsh and vulgar. Fortunately there is plenty of it offering ample room for filtering and curveshaping.

The number of delaying elements does not have to be just 6. If you like, another (or more) "4050" may be employed between the F-gate and the transistor base. The propagation of the crawling action is dependent on the value of the capacitors and will be slower if bigger capacitors are in use.
Two invertergates may replace one buffergate -- worth to take into consideration, if you have plenty of inverters but no buffers.
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