Alright, alright already! I know this is what some people have been waiting for - actual freakin' construction. So I'll just get straight to it.
It was hard to find a 2.1mm DC jack that wouldn't need me to drill in to my board to create slots that the terminals could be inserted in to for soldering (i.e. nothing with pins on a 0.1" grid), but I had a flash of inspiration and figured that a surface mount jack with some loops of wire over the top of its terminals could be used to make my own "through hole" socket, so thats what I did, and this is what I ended up with for the power supply:
Quite pleased with that result, and the DC jack feels very solid. I used the off cuts of the rectifier diode leads to form the loops as they were quite hefty.
Next came a sprinkling of other building blocks and bits and pieces: the power on reset RC circuit, the AC_CLK/ generator and isolation switch, reset button, and a handful of transistors forming a few gates of logic to glue all of that together.
Soldering all of that in to place, the board now looked like this:
Then I started soldering even more transistors to start building the prescaler.
And around 100 transistors later and after much effort soldering the many many fiddly wires, I was left with this:
The 8 pin socket on the back forms a bus to the next board to carry power and the MCLR, AC_CLK/ and 1PPS_CLK/ signals over.
Confession: construction of the analogue board started in early March, and a couple of things have changed in between then and now, including the AC_CLK/ generator and some circuitry related to the reset button which provides some (better) hardware debouncing. So if you're wondering why perhaps the components you see don't quite line up with the schematics I posted recently, that would explain it. The schematics represent how it has actually been (finally) built.
The thinner blue, red and black wires are 0.22mm2 or 30AWG Kynar insulated wires. They are also sometimes known as "wire wrapping wire".
And here's another slightly artsy shot, because I couldn't help myself.
So yeah, thats about it for construction of the analogue board. The next step was to test it and figure out if this was going to work. And this was where I learned my first lesson: don't build too much all at once.
The power on reset delay circuit was also easily testable by probing the drain pin of its transistor while applying power. When power is applied you see the output high, and after a short period of time while the capacitor charges up it goes low.
That feeds in to the reset mux, along with the reset button, so testing that was a matter of probing the output of the inverter that sits after the NOR gate to which the power on reset delay and reset button are connected. The inverter outputs a positive logic signal referred to as MCLR, and it is high when ever the reset button is pressed, or when the capacitor in the power on reset delay circuit is charging just after power on. That also checked out OK.
Testing the AC_CLK/ generator is a little more complex, I really need to see the waveform it produces alongside the input AC sinewave since it happens too fast to see on a multimeter, so this needed an oscilloscope.
Backstory: I'd been hoping to win something as part of Keysight's Oscilloscope month promotion, but alas I wasn't so lucky. Towards the end I caved and ended up buying myself a Keysight "premium used" MSOX2024A which ended up costing me about half what a brand new model would have cost. I have to say I'm incredibly impressed so far, it looks for all the world like a brand new scope, and came with 5 years warranty. Highly recommend you check out this route first if you're looking to buy a Keysight... So yes, some of the material for this post dates back to then and even earlier. I also started a new job within the past month and a bit, so that has kind of slowed things down a bit too.
Using my newly acquired scope I took some measurements of the rise and fall times of the output of the AC_CLK/ generator to see what its waveform looked like. A very early version of this part of the circuit used a BC547 and I wasn't happy with the output, in particular that the output voltage was only around 4.5V which was uncomfortably close to the gate threshold voltage. So to be safe I re-designed it to use a MOSFET instead, and was much happier with the output voltage at 9V. The rise and fall times were nice and sharp at no more than about 15uS (microseconds) at worst (fall time was just a smidge over 3uS) - also much faster than the original BC547 based circuit. Here are some scope traces of that:
That is of course looking at the time taken to transition between ground and peak voltage - actual "apparent" rise/fall time would be much quicker, because the transistors don't need the full 9V to turn on or off, that happens closer to 4.5V.
The output of the AC_CLK/ generator feeds in to a NOR gate along with the MCLR signal, such that when ever MCLR is asserted the AC_CLK/ signal is held high. When MCLR is not asserted, the AC_CLK/ signal is free to pass through and oscillate. So testing that was a matter of probing the output of the inverter that forms the output of that logic. This was also easy to test with a multimeter in frequency mode, I got ~50hz, so that is all working fine.
On a roll I thought.
Then it came to the prescaler. Was I going to get lucky and was it going to work at all? All of the simulations and breadboarding said it should, but now we're in the real world and at bigger scale.
I used my multimeter in frequency mode to probe the output of the first stage. It was giving me 25hz which is what I expect (since every stage of the prescaler acts as a divide by 2 of its input frequency. The second stage was giving me 12.5hz - brilliant. And the 3rd stage was giving me 6.25hz. "Amazing" I'm thinking, "did I get this all in one go?"
Then I probed the output of the 4th stage and nothing. Zip. Nada. Ah crap, my luck had run out. Since I didn't actually have my oscilloscope at this point in time, I fabricobled a crude logic probe out of an LED, transistor, and a couple of resistors to allow me to get a quick visual from the circuit:
If you would like to build your own, here's the schematic of what I built (you may need to adjust the value of R2, I picked 2.2K to work with the 9V of my power supply, but other logic circuits might need something a little less, perhaps 1K for a 5V circuit for example):
Using this I probed around at the various inputs and outputs to the 4th stage to see where the signal was getting lost, and it just happened to be a solder joint that hadn't properly wetted from the source pin of one transistor to the ground rail. A tiny bit of reflow sorted that right out, and the 4th stage was giving me 3.125hz. Bingo. That was where lesson #1 was learned, because the solder joint was buried in between the wiring, it was a bit tricky getting down in there to make the repair. For future portions of the build I decided I would build and test counter stages one by one to avoid problems like that.
The 5th stage gave me 1.5625hz, and finally somewhere around 0.78125hz at the 6th stage (logical conclusion, because my multimeter can't display that many decimal places). Great, they are all working.
I then proceeded to hook the second, fifth and sixth stages (which represent BCD values of 2+16+32) up to the 3 input NOR gate which feeds the reset circuitry, and probed the non-inverting output of the SR latch, and what do you know, I was getting a very brief pulse once every second.
And with that, the analogue board is complete! It is producing all of the necessary clocking and other signals that are needed to move forward to the next board which will be the "time module", counting seconds, minutes and hours, so I hopefully shouldn't need to pay any more attention to this one.
Build
I started with construction of the power supply, since that was going to be rather necessary. For a slight retro look I decided to use an axial capacitor as the smoothing capacitor on the input to the linear regulator. The full bridge rectifier has a 100nF capacitor in between the DC terminals as I read this can be good for filtering noise at the output of the rectifier. This then feeds via the smoothing cap in to the 7809 regulator. Another 100nF cap at the output of the regulator helps to filter noise on the output here too (not that I expect that much noise, but it can't hurt) and the positive rail then feeds through a PTC (fuse) for short circuit protection - theres a lot of fiddly wiring all done by hand, and if I happen to short something out or it breaks loose, I don't want anything getting damaged.It was hard to find a 2.1mm DC jack that wouldn't need me to drill in to my board to create slots that the terminals could be inserted in to for soldering (i.e. nothing with pins on a 0.1" grid), but I had a flash of inspiration and figured that a surface mount jack with some loops of wire over the top of its terminals could be used to make my own "through hole" socket, so thats what I did, and this is what I ended up with for the power supply:
Quite pleased with that result, and the DC jack feels very solid. I used the off cuts of the rectifier diode leads to form the loops as they were quite hefty.
Next came a sprinkling of other building blocks and bits and pieces: the power on reset RC circuit, the AC_CLK/ generator and isolation switch, reset button, and a handful of transistors forming a few gates of logic to glue all of that together.
Soldering all of that in to place, the board now looked like this:
Then I started soldering even more transistors to start building the prescaler.
And around 100 transistors later and after much effort soldering the many many fiddly wires, I was left with this:
The 8 pin socket on the back forms a bus to the next board to carry power and the MCLR, AC_CLK/ and 1PPS_CLK/ signals over.
Confession: construction of the analogue board started in early March, and a couple of things have changed in between then and now, including the AC_CLK/ generator and some circuitry related to the reset button which provides some (better) hardware debouncing. So if you're wondering why perhaps the components you see don't quite line up with the schematics I posted recently, that would explain it. The schematics represent how it has actually been (finally) built.
The thinner blue, red and black wires are 0.22mm2 or 30AWG Kynar insulated wires. They are also sometimes known as "wire wrapping wire".
And here's another slightly artsy shot, because I couldn't help myself.
So yeah, thats about it for construction of the analogue board. The next step was to test it and figure out if this was going to work. And this was where I learned my first lesson: don't build too much all at once.
Testing
Testing the power supply was easy with a multimeter, and the results predictable. 12V AC in, 22V after rectification and smoothing, fed in to the linear regulator which produced 9V DC out.The power on reset delay circuit was also easily testable by probing the drain pin of its transistor while applying power. When power is applied you see the output high, and after a short period of time while the capacitor charges up it goes low.
That feeds in to the reset mux, along with the reset button, so testing that was a matter of probing the output of the inverter that sits after the NOR gate to which the power on reset delay and reset button are connected. The inverter outputs a positive logic signal referred to as MCLR, and it is high when ever the reset button is pressed, or when the capacitor in the power on reset delay circuit is charging just after power on. That also checked out OK.
Testing the AC_CLK/ generator is a little more complex, I really need to see the waveform it produces alongside the input AC sinewave since it happens too fast to see on a multimeter, so this needed an oscilloscope.
Backstory: I'd been hoping to win something as part of Keysight's Oscilloscope month promotion, but alas I wasn't so lucky. Towards the end I caved and ended up buying myself a Keysight "premium used" MSOX2024A which ended up costing me about half what a brand new model would have cost. I have to say I'm incredibly impressed so far, it looks for all the world like a brand new scope, and came with 5 years warranty. Highly recommend you check out this route first if you're looking to buy a Keysight... So yes, some of the material for this post dates back to then and even earlier. I also started a new job within the past month and a bit, so that has kind of slowed things down a bit too.
Using my newly acquired scope I took some measurements of the rise and fall times of the output of the AC_CLK/ generator to see what its waveform looked like. A very early version of this part of the circuit used a BC547 and I wasn't happy with the output, in particular that the output voltage was only around 4.5V which was uncomfortably close to the gate threshold voltage. So to be safe I re-designed it to use a MOSFET instead, and was much happier with the output voltage at 9V. The rise and fall times were nice and sharp at no more than about 15uS (microseconds) at worst (fall time was just a smidge over 3uS) - also much faster than the original BC547 based circuit. Here are some scope traces of that:
That is of course looking at the time taken to transition between ground and peak voltage - actual "apparent" rise/fall time would be much quicker, because the transistors don't need the full 9V to turn on or off, that happens closer to 4.5V.
The output of the AC_CLK/ generator feeds in to a NOR gate along with the MCLR signal, such that when ever MCLR is asserted the AC_CLK/ signal is held high. When MCLR is not asserted, the AC_CLK/ signal is free to pass through and oscillate. So testing that was a matter of probing the output of the inverter that forms the output of that logic. This was also easy to test with a multimeter in frequency mode, I got ~50hz, so that is all working fine.
On a roll I thought.
Then it came to the prescaler. Was I going to get lucky and was it going to work at all? All of the simulations and breadboarding said it should, but now we're in the real world and at bigger scale.
I used my multimeter in frequency mode to probe the output of the first stage. It was giving me 25hz which is what I expect (since every stage of the prescaler acts as a divide by 2 of its input frequency. The second stage was giving me 12.5hz - brilliant. And the 3rd stage was giving me 6.25hz. "Amazing" I'm thinking, "did I get this all in one go?"
Then I probed the output of the 4th stage and nothing. Zip. Nada. Ah crap, my luck had run out. Since I didn't actually have my oscilloscope at this point in time, I fabricobled a crude logic probe out of an LED, transistor, and a couple of resistors to allow me to get a quick visual from the circuit:
If you would like to build your own, here's the schematic of what I built (you may need to adjust the value of R2, I picked 2.2K to work with the 9V of my power supply, but other logic circuits might need something a little less, perhaps 1K for a 5V circuit for example):
Using this I probed around at the various inputs and outputs to the 4th stage to see where the signal was getting lost, and it just happened to be a solder joint that hadn't properly wetted from the source pin of one transistor to the ground rail. A tiny bit of reflow sorted that right out, and the 4th stage was giving me 3.125hz. Bingo. That was where lesson #1 was learned, because the solder joint was buried in between the wiring, it was a bit tricky getting down in there to make the repair. For future portions of the build I decided I would build and test counter stages one by one to avoid problems like that.
The 5th stage gave me 1.5625hz, and finally somewhere around 0.78125hz at the 6th stage (logical conclusion, because my multimeter can't display that many decimal places). Great, they are all working.
I then proceeded to hook the second, fifth and sixth stages (which represent BCD values of 2+16+32) up to the 3 input NOR gate which feeds the reset circuitry, and probed the non-inverting output of the SR latch, and what do you know, I was getting a very brief pulse once every second.
And with that, the analogue board is complete! It is producing all of the necessary clocking and other signals that are needed to move forward to the next board which will be the "time module", counting seconds, minutes and hours, so I hopefully shouldn't need to pay any more attention to this one.
Very cool! Even though I have only glanced over it yet. I'm a bit atm, but will definitely come back to this asap. Keep it coming, man!
ReplyDeleteIts a bit of a big one. Design files for the time module are now up, too. :-)
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