My new CRT driver board is coming along rather nicely. Tonight I tested it out with a 5″ CRT. It uses a P7 radar phosphor so it looks bluish white with a sickly yellow persistence.
The pattern is a Lissajous figure (LISS-uh-joo). Take two waveform generators and connect one to the X input and the other to the Y input, and you get all sorts of interesting patterns. Since the CRT driver board is not available as a kit (not yet, anyway!) you can duplicate this with an oscilloscope and two function generators.
There’s some interesting math behind Lissajous figures, but I’m more interested in building 3KV power supplies.
At Maker Faire, a lot of people asked me if I had a kit available for any of my CRT clocks. Based on the amount of interest, I’ve decided to put together a kit that will make it easy for people to drive cathode ray tubes using simple digital or low voltage analog control signals. The kit will include a PC board and all the components as well as detailed assembly instructions. For people that opt to use the digital interface, the kit will also include source code libraries making it easy to generate simple vector graphics.
The kit will use surface mount components, but none smaller than 0805. The ICs will be SOIC or SOTs, with the exception of the DAC, which is TSSOP.
Because this would be the very first surface mount kit many people attempt, I’m trying to figure out an approach for the assembly instructions that will make it easy to succeed. Some ideas I’ve had so far are:
Solder the DAC first since it has a fairly fine pitch package (TSSOP). The kit might include a second DAC as a spare. By soldering it first, it’s easier to check for short circuits and open circuits. Another approach is to make a “spare parts kit” available that has some of the commonly “blown” parts.
Assemble the kit in sections, testing the circuit a piece at a time. For example, after assembling the DAC, you would assemble the filament power supply and then test it to make sure it works and outputs the proper output voltage. This makes it easy to correct any mistakes as they occur. I don’t want people to assemble the whole board, throw the switch, and not have a working kit–or worse yet, have the kit go up in smoke.
It makes sense to release the assembly instructions on a site like Instructables, where it’s easy to include detailed macro photos of critical assembly details (like diode orientation). It also makes it easier to correct the instructions for mistakes, and it avoids the environmental impact of including printed instructions with the physical kit.
Hobbyists seem to have an aversion for surface mount components. With a little practice, I’ve found that it’s faster and easier to use surface mount components. Think about all the time you could save by not having to bend and clip resistor leads. You can solder most of the components without having to flip the board over.
If you have any ideas, please feel free to comment. This is all still in the early stages so there is plenty of room to change things and try new approaches.
The Maker Faire is a neat DIY convention that happens every year. I’m bringing some of my projects to the Maker Faire Bay Area; just look for Tube Time. Come and say hello!
Watch this YouTube video, and then read the rest of the post.
So how did I do it? It is actually a very simple circuit.
The LM1881 separates the sync signals from the NTSC composite video coming from the camera. It outputs a vertical sync signal (active low) that asserts during the vertical retrace period and a composite sync signal (also active low) that asserts during the horizontal retrace period and also during the vertical retrace period (but with a set of serration and equalization pulses).
To connect these to my oscilloscope, I have to use the XY mode on the scope and convert the sync signals into deflection signals. This is done using analog ramp generators. The simplest way is to use an RC circuit to generate a rather nonlinear ramp. When the sync signal goes high, it charges the capacitor through the resistor. When the sync signal goes low, the diode allows the capacitor to discharge immediately. This generates the sawtooth waveform. Adjust the R value so you get the most complete ramp (goes most of the way up to 5V).
The video signal is fed directly into the Z-axis signal at the back of the scope. Because the Z-axis signal has the opposite polarity from regular video (it is a blanking signal, where a positive voltage will turn the beam off), I had to build a really basic video buffer to invert the signal. This is a nice exercise in transistor biasing using four external resistors. Don’t ask me for the schematic–you should try to build it yourself. Even if you don’t get it working properly right away, you’ll discover all sorts of interesting analog video effects!
Yes, it’s not really vacuum tube related, but I built an entry for the 555 timer contest. It uses an ICM7555, which is Maxim’s second source of Intersil’s CMOS version of Signetic’s original NE555 timer. Turns out the fact that it is CMOS is important for this particular circuit…
My entry is an AM radio. The only active device (silicon, germanium, or otherwise) is the ICM7555. The tuning is accomplished with an inductor and a capacitor, and the ICM7555 acts as an AM demodulator and class-D power amplifier to drive the speaker.
You may be wondering how all this is accomplished with a 555. The schematic is below.
Here’s how the circuit works: The AM radio signal is tuned by inductor L, which is 300 turns of wire on a 1/2 inch diameter cardboard tube made out of an old toilet paper roll, along with the 100pF variable capacitor. One end of the parallel configuration of L and C connects to an antenna (surprisingly long!) and the other end connects to a ground wire which is tied to the AC outlet ground (old books tell you to ground it to a water pipe). So far this is exactly like an AM crystal radio.
The 555 timer is configured as a pulse width modulator in a non-traditional configuration. If I used the standard approach and connected the input to the CV pin, the low impedance of the pin would prevent the circuit from receiving any radio signals. I had to invert the circuit and tie both high impedance analog pins, Threshold and Trigger to the radio signal input. This is the reason why the CMOS version of the 555 timer performs much better than the standard bipolar, which has higher input bias current.
The pulse width modulator ramp is created by the 0.01uF capacitor and the 10K bias potentiometer which are connected to the Discharge pin. The potentiometer wiper goes to the LC arrangement. With no radio signal coming in, the voltage on Threshold/Trigger ramps up until it hits the threshold, and then Discharge causes the voltage to ramp down again.
When a radio signal comes in, it gets superimposed on the ramp signal, causing the threshold and trigger comparators to trip early or late in a cycle. This variation causes the output duty cycle to vary, which we can hear as sound in the speaker.
Demodulating the signal properly requires adjustment of the bias knob, so that part of the radio signal is “clipped” and ignored by either the threshold or trigger comparators. This ensures that the negative “halves” of the radio wave don’t cancel out the positive “halves”.
Want to hear what it sounds like? Check out the video below:
And of course, I can’t end the post without a gratuitous shot of the ICM7555 in circuit.
