[Jon] wanted his speakers to come on and off along with his TV. The speaker heats up if left on so he didn’t want to do that. But killing the power also resets the volume level (this is an old set of PC speakers and the remote is wired, not IR) so using one of those switched power strips was out as well. He thought a bit about trying to use the power LED on the TV to build his own circuit when it dawned on him. It’s possible to monitor the USB port on the TV and use it to switch on the speakers.
The circuit above uses a couple of opto-isolators to protect both the television and the speakers. The 5V line from the USB port on the back of the TV is monitored by an XNOR gate (which helps to filter out some of the toggling at power-on). When that gate latches it activates a 555 timer which in turn fires up the speakers. Presumable this happens when power is cut as well, but we’ll let you work through the circuit logic yourself.
If you’re attempting to debug a serial bus with a bare-bones logic analyzer, you’re going to have a bad time. Most of the inexpensive analyzers available don’t have a serial pattern trigger, or a way to start recording data after a specific pattern of bits comes down the pipe. [Neil] sent in a great little project that adds a serial trigger to these analyzers, we’ve got to hand it to him for designing such a useful board.
[Neil] designed a small board featuring a CLPD that converts serial data to parallel data. By setting the trigger condition of the logic analyzer to any 24-bit pattern he wants, it’s possible for [Neil] to sniff a serial bus exactly when he wants to.
The circuit is quite minimal, basically just a 100-pin CLPD and a bunch of 0.100″ header pins. It’s a useful tool, and although we couldn’t find the board file to make our own, we’re sure [Neil] will be providing that shortly.
Motion sensing can be quite effective when taking photographs of wildlife. But how can one be sure that the motion was at the center of the frame? A PIR sensor picks up motion in its entire viewing range. It’s not really something that can be aimed. But if you use two PIR sensors you can monitor a focused area for motion.
The trick is to use a logic circuit. By building an AND gate you can trigger based on motion in the area which is overlapped by both of the sensors. In this case the AND gate is built from a voltage divider. The outputs of the PIR sensors are connected above and below the divider’s connection to the photo trigger. Both have a protection diode, and the divider is tuned so that both PIR outputs must come one in order to raise the trigger input above the voltage threshold. So much for never crossing the streams.
[Alex] was tasked with a control design problem for a set of motors. The application called for the back of a truck to open up, some 3D scanning equipment to rise from its enclosure, and finally the equipment needed to rotate into place. All of this needed to happen with one flip of a switch, then proceed in reverse when the switch was turned off. We can understand why the final design used a microcontroller, but we also think that [Alex’s] relay logic circuit is an eloquent way of doing things.
He uses limiting switches as the feedback loop for the logic. In the video after the break he walks us through the schematic. Each of the three motors has an up and down limiting switch. These control the three relays which switch power to the motors. We like the design because interrupting the movement mid-operation provides no problem for the system. The only real issue we see is that relays wear out, and the automotive application of the hardware may cause this to happen more quickly than normal.
You may recognize the clear gears used in the demo. [Alex] previously showed us how he makes those.
Continue reading “Mechanical relay logic that was snubbed for a microcontroller”
If you’re going to learn digital logic, why not aim high? That’s what [Easton] and his friend did when they built a clock using only 4000-series logic chips. On a breadboard, no less.
For a 1 Hz clock, [Easton] and his friend used a 4060 counter paired with a flip flop. This counts off 59 seconds until, with the help of an AND gate, the seconds counter rolls over to zero. After repeating that again for the minutes and building a similar circuit for the hour, and [Easton] had a working 4000-series 24-hour clock.
The breadboard clock may not be the prettiest thing, or a textbook example of how to prototype circuits, but that was fixed with [Easton]’s friend’s PCB layout of a 12-hour clock. We couldn’t find any pics of this, but we’re sure it’s awesome and a great way to learn about logic and design.
[Kyle] has been hard at working building an 8-bit computer from the ground up. He’s using a set of logic IC’s for the various components, and some NVRAM chips to store the control words. What you see above is the roadmap for his instruction set. He’s just started writing them to the chips, making the job easier by building an Arduino-based programmer.
We’ve enjoyed watching [Quinn Dunki’s] progress with her
Z80 6502-based PC build which started on a breadboard in much the same way but has come a long way since those humble beginnings. Recently we also looked in on a 4-bit computer that is using discrete components. But [Kyle’s] take on the challenge falls somewhere in between the two.
The gist of his design can be found in one of his earlier post. He’s got a ring counter which starts by clearing the address register. It then loads the NVRAM address of the next instruction which is then executed on the subsequent count. It seems the build still has some way to go so make sure to keep your eye out for updates.
[Andrea] built this LED chaser using one logic chip. It illuminates all but one of the six LEDs, with the dim bit moving back and forth along the row in a chase sequence. This is something like an inverse Larson Scanner without the fading tail. But doing it with a logic chip instead of a microcontroller is a fun challenge.
Which brings us to the point of this feature. [Andrea] didn’t really post an explanation of how the circuit works. Usually missing details mean that we archive the tip and move on to the next one, but we think this provides a fun activity. Can you figure out how the circuit works? We already know that it’s using a CD4017 decade counter/divider chip. This gets its clock signal from a 555 timer circuit. [Andrea’s] schematic is a bit hard to read, but grab a copy, blow it up a bit (or use your browser zoom) and study the CD4017 datasheet (PDF) if you need to.
Want proof that it does actually work? It’s embedded after the break.
Continue reading “Challenge: Figure out how this logic-based chaser works”