Project a3 hack

One servo for X and one for Y is all you need. Another microservo lifts the pen up and down using a hinge you could also get from a hardware store. Is it pretty? Does it do amazing artwork? No, again. But it is the kind of thing you could probably throw together from things you happen to have hanging around, especially if you are about to trash an old desk or cabinet with slides in it.

This would make a great rainy day project. We are suckers for simple plotter projects even though you could just mate a pen to your 3D printer or CNC machine. It is interesting. Sometimes the whole point is to learn about the theory behind how the device works through practical experimentation.

Building it means you get a real-world feel for what limitations a particular mechanical design has. That in itself, is a worthwhile goal for a project. The fact that this is just done with off-the-shelf mundane parts is a bonus as those same parts can be used for something else when the experiment is finished. Building something for fun has value in itself.

Totally agree. For me, i often build similar stuff to just pass time. They end up totally useless but I still feel accomplished. Doing something with hands and brain lets you relax. It does a very good job of demonstrating a generic X- Y-Z machine — cheaply. In my eyes that makes it a winner. I dig this. Like if you gave me a better sketch from a better pen plotter, I could paint ya something convincingly realistic.

This thing? For Deghosting multiplexed LEDs. LEDs also act as tiny capacitors and this can be a source of them lighting up when you don't want them to. Blinking LEDs, what can be easier, right? For a recent project I needed to multiplex 2 sets of 6 LED each. The circuit schematic has the LEDs driven by 6 pins on an Arduino plus two pins to activate one bank or the other.

My Arduino code lights up each bank of LEDs for around 1ms with its corresponding pattern. This sounded simple enough. As stuff is happening every 1ms, first thing I did was slow it down.

I went down to 1 second between changing banks to see exactly when the ghosting occurs. The timing diagram shows the transition from displaying LED After turning off both banks there is a delay to account for the transistor switch-off time and then all LEDs are switched off.

Everything was already off so nothing changed here. My Arduino code lights up each bank of LEDs for around 1ms with its corresponding pattern. This sounded simple enough. As stuff is happening every 1ms, first thing I did was slow it down. I went down to 1 second between changing banks to see exactly when the ghosting occurs. The timing diagram shows the transition from displaying LED After turning off both banks there is a delay to account for the transistor switch-off time and then all LEDs are switched off.

Everything was already off so nothing changed here. Then line A0 goes high because in the next bank LED7 should be on. At this moment LED1 flashes briefly. Afterwards the bank containing LED7 is turned on by bringing line D3 high. Simplifying the schematic just to the minimum number of components we need we can look at the steps the circuit goes through. Only when A0 goes high LED1 flashes briefly. Were the transistor leaky, LED1 would stay dimly lit, not just flash.

So it looks like a capacitor is charging. This suggests that LED2 behaves like a capacitor and a little bit of current goes through it until it charges. John suggested an alternate solution that requires no changes to the software and no extra MCU pinL Precharge the LEDs by just connecting a fairly large-valued resistor from Vcc to the common line.

I tried it out with a couple of 20k resistors and it works! Paul suggested an alternate solution, namely to turn off LEDs by setting them to high impedance instead of 0V. That way the tiny LED capacitances have no way to charge and thus no ghosting. I tried it and it works! Awesome "zero extra components" solution, fixing it in software. View all 2 project logs. Create an account to leave a comment.

Log In. Also, your 1st video shows the dim LED on the whole time even while camera is moving over time, is the tranny really off, or your video is not really correct? If it is bleeding I have one solution. Are you sure? Naa, not phosphor decay I've seen red ones do that too i haven't tried with other colors cause I've never used them.

Then something is wrong with your circuit design or multiplexing algorithm. LED multiplexing is very old, it goes back at least to the one chip clocks and calculators of the 70s. You'd better hop on a time machine to tell those designs that they are supposed to have ghosting. Also tell the multiplex designs I have built. When you are working at that kind of speed, comparable with the instruction times of a MCU, then the order of operations in the firmware becomes more important.

For example for a numeric display the correct way is to turn off the current digit drive line first, change the segments, then turn on the next digit. Even the old chips took care over this point. Did you try charging the capacitance via a resistor connected between VCC and the collector of the transistor. Make its value high enough not to impact the voltage across the LEDs but low enough to quickly charge the capacitance of the LEDs when the transistor is in its off state.

Hi John!