Complex 555 based frequency or PWM generator

[00:00:00]This video is strictly about this little pulse generator circuit that uses a 555 in a really odd way because it fits two functions in that has a very strange bit of circuitry.

[00:00:11]And this is actually for controlling stepper motors. I made a video about the stepper motors and how to connect them to the drivers, but I'll give you a quick demo this really quick and then we can get straight into the circuitry. So, I'll set that up right now.

[00:00:25]So, the module is currently sending a stream of pulses over to this driver which then drives the stepper motor.

[00:00:31]If you vary the knob here, it varies the speed of the stepper motor and you've got three speed ranges.

[00:00:38]If I stop it and put it to its lowest speed range which is set by a link here which I'll show you on the schematic and the actual close up of this.

[00:00:46]When you turn it on, it turns extremely slowly and that's its lowest setting. This is its high setting, but there is also another setting we can go to which is the super turbo setting, the high speed setting.

[00:00:59]And if we set it to that, the slowest speed is that, but it goes right up to the point that the stepper motor becomes a blur.

[00:01:09]But it loses its torque when it goes there, but you can adjust that. You can adjust the current in here.

[00:01:13]Uh one other thing you can do is you can press this button and it reverses the direction, so I can reverse it backwards or forwards and it also has the enable button. There is another function this that puts out pulse width modulation, but all that happens when you actually use that function with a stepper motor is you get very little a very slight change, but other than that, just not really much. It just goes at more or less the same speed, not much variation at all, but that's not what it's intended for. Okay, let's take a look at the circuit board.

[00:01:45]So, the circuit board has a power input rated for 8 to 24 volts and it's rather disappointing given that this uses very little current. It's disappointing that they've not used a polarity protection diode in this, particularly cuz it goes straight to this capacitor. So, you could blow the board up in short measure. If you do, you can actually will either fix it or you can actually just run it from 5 V from this edge connector.

[00:02:08]It has a 7805 voltage regulator to convert the output from the input to 5 V. And then it's got a 500 mA electronic fuse. I think that's 500 mA.

[00:02:22]And then a classic NE555. And I thought, "Oh, this is going to be easy to reverse engineer." No, it's not.

[00:02:28]Uh because it does something very odd.

[00:02:30]It also skewed me because the two modes, the frequency mode, which is the bottom link here, and the pulse width modulation mode, which I guess it's maybe for servos or something like that.

[00:02:41]Um they use some very odd circuitry. I thought it was two distinct sections, particularly cuz this potentiometer has two separate potentiometers. It's almost like a stereo one. But I'm guessing the reason they use that is just for the extra strength and ruggedness of like basically having six pins instead of three. Or maybe they originally intended to do it a different way.

[00:03:02]So, let's start off with some simple stuff. This is the speed range selector.

[00:03:07]And all it does is it switches the capacitor. These capacitors are effectively all connected to the timing pin of the 555, but it switches them individually to the 0 V rail. And therefore, whichever one it's in, that is the timing range. Theoretically, you could sort of put a boundary combination in, but to be honest, it's such a wide range that it wouldn't really have much effect.

[00:03:28]This link here, if you put the uh link the little link over here onto these pins, the top one is pulse width modulation.

[00:03:38]Not seen a use for that yet, but the bottom one is what you usually have it on for a variable frequency string of pulses out the unit.

[00:03:45]This is a double pulse changeover switch. One of them is used to alternate the control signal. In this case, this is the direction one, so it's the direction output here.

[00:03:56]It alternates it between positive and negative with a 100 ohm resistor in series just to limit fault current. And likewise, the enable alternates between positive and negative as well.

[00:04:07]Um the output from the 555 is swinging positive and negative by default. So, it is sort of like it can be used with reference to 0 volts or 5 volts. They've got a little four-pin connector here which has the 0 volts on it. I am my test just the 5 volt supply.

[00:04:22]It's worth mentioning enable, direction, and clock or pulses as marked in the stepper motor driver.

[00:04:30]Um and then they've just put these in Chinese so they're marked. They're I've marked them in color. So, that's the 0 volts and that's the 5 volts. Not sure why they switched to particularly when they've grown over there.

[00:04:40]But they left in Chinese. Interesting.

[00:04:43]Right, well tell you what, I think that's me covered just about everything.

[00:04:46]Um one other thing I could mention is these double pull switches. Uh the other switch actually switches just used to switch an LED on in both instances just to show them what it's in. Okay, let's take a look at the schematic.

[00:04:59]And the schematic uses quite unusual circuitry here and it had me trying to work out what it was doing particularly because the circuit board is double sided with tiny little pads for probing on and you're having to probe both sides at once. And also, it's black. It's got the black solder resist which makes it harder to copy, but doesn't stop you completely.

[00:05:18]So, there's the incoming supply minus the polarity protection diode which would have been great. There's a capacitor I didn't actually note the value of that.

[00:05:26]Uh 100 microfarad 100 microfarad 35 volts.

[00:05:34]Then that goes straight to the uh 7805 regulator where these capacitors I guess they're 100 nano. That's pretty typical for the regulator. And then I've just shown it as an actual fuse, but it's the self-resetting PTC fuse just to protect against little incidents.

[00:05:50]It won't protect against the diode and input instant if you connect polarity wrong.

[00:05:54]There is the power indicator LED. Quite a high value resistor, but it's only going to pass a fraction of milliamp through the LED.

[00:06:01]And then we have this complex circuitry.

[00:06:03]So it it kind of it can be used the conventional way 555s are normally used, but it can also be used in the style of pulse width modulation. Um so when you set the links and these are the two links that set the mode there.

[00:06:16]If you set it to the frequency mode, um then the Let me think here. Yes, if you set to frequency mode, uh you effectively have the capacitors are charging via this resistor, uh through the shunt, through this variable resistor, then through this completely unnecessary resistor onto the it charge the capacitors up that are monitored by the two thresholds, the upper threshold and the lower threshold. When the charge is up to the upper threshold, it turns on the pin seven's discharge resistor, and then it discharges through the same resistor. That's where the pulse of modulation is different cuz it actually charges via a different the other First, I'll come back to that in a moment cuz uh let's finish what I was doing in the first place.

[00:07:04]Here are the three capacitors, 1 nano, 10 nano, and 68 nano measured in circuit, closest I could get them. And there's the links that choose. 10 nano is a good typical starting volume.

[00:07:15]Uh when you switch the link here from frequency to pulse of modulation, now what happens is that when the uh capacitor charge up, it comes via these resistors through that diode, through this side of the potentiometer, and then uh does the thing where it charge the capacitor up to the threshold. But then when the discharge resistor comes on, it's going to discharge through this uh diode, and that means it's the other side of potentiometer, so depending on the position it's going to give you effectively a pulse and modulation effect. The one side will be the on and the other side will be the off and by varying the potentiometer you change the mark space ratio.

[00:07:56]And the outputs, that's one of these things I've just described it and it sounds so easy but reverse engineering that weird circuitry was not easy.

[00:08:06]Here's another thing. The output pin three, pin three I'll write it there.

[00:08:11]And is going to the clock signal output >> [snorts] >> via the 100 ohm protection resistor to protect against short circuits. If it does get short to ground, 5 volts to the zero volt rail would be about 50 milliamps times 5 volts quarter of a watt. So these resistors are just basically on their limit if there is a fault but ultimately that's the way time they're going to be in situation. What is odd is this one key pull up resistor because the 555 doesn't need that. It actually pulls up to the 5 volt rail itself. Maybe they intended I don't know why they put that resistor in there.

[00:08:46]But anyway, when you use the switches that select direction whether it's going to be positive or negative and enable when it's going to be positive or negative, it is just the double pole double throw switches but one side is switching between the positive rail. I use this red line just to keep it clearer that this is also plus 5 volts.

[00:09:06]And it switches each of those outputs between the positive and negative in both instances. But also the indicator LEDs, they are on the other half of those switches and it's just basically switch it to the zero volt rail to light the LED.

[00:09:20]And that is it. So easy to explain but that was a multiple visit reverse engineering. That was not actually easy to reverse engineer because the weird circuitry here and and the way it was all selected from the side by links.

[00:09:35]But there we have it. It's a useful little module if you need something that provides a variable output frequency string of pulses. And you do have the option, if it doesn't suit you the frequency range it covers, you could potentially put your own capacitors in here to nudge things.

[00:09:51]But that's it. An interesting little module.