throbscottle
Well-Known Member
So, this is what I have so far for the new PCB drill. The feed control is partially tested - until the 7474 arrives I'm having to substitute a 4013 for a 7474, real head-scratcher until I realised the r/s inputs are non-inverting on the 4013! Motor control works correctly as far as timing and detecting the feedback pot position is concerned, I still need to test the actual feedback circuit.
I hope no-one is going to say "why don't you just use a microcontroller" - I wanted to test my circuit design skills!
So, the feed mechanism is the head positioning mech from a cdrom drive, I stripped out the optics and fastened the drill motor to the carriage with some steel wire. I fixed a slotted photo interrupter to the chassis so it detects the teeth of the final gear. The output of the phototransistor sits at just under 5v, so it's feeding the gate of a p-channel mosfet (Q1), which switches pretty well. Needed to put in a level shifting resistor to pull the input down a little. The mosfet feeds an npn so there is a place for other signals to feed in, and that feeds into the up/down control of a digi-pot.
There is a 1/2 556 monostable which generates a pulse to allow the digi-pot to change direction. It gets it's direction signal from flip-flop Q*. C11 creates a bit of lag to ensure the digi-pot's c/s input returns to 0v whilst the direction signal is still present.
When power is applied, Q4 holds flip-flop 1 in "set" until C12 is charged. This runs the motor until the mechanism reaches the top, where there will be a microswitch. This turns on Q3, fully charging C12 so the motor stops, and via 1/4 of the 4066 feeds pulses from the pwm into the digi-pot and winds it up to the top end. Then it sits and waits for a press on the "go" button (the plan is to use a footswitch for this).
When the button is pressed, it clocks a 1 into the 2nd flip-flop, sending the motor on it's way down to the lower set point. The comparator is wire-or'd with the output of the 1st flip-flop, so it turns off the motor before the 1st flip-flop gets it's clock input, which is delayed by C13 to allow for motor over-run. The 7474 has Schmidt trigger clock inputs so is happy with the slow rise (though the 4013 doesn't but is still OK). When the the clock signal arrives, the motor runs the drill back up until it reaches the upper set point. This resets the 1st flip-flop and stops the motor until the button is pressed.
The pulse to change over the digi-pot is obtained by detecting when the 2nd flip-flop changes state. Either C8 or C9 will always be charged via R23 or R25, connected to Q and Q*. When the outputs switch over, this is discharged through it's diode. The other cap then charges through it's resistor, slow enough to create a brief low pulse to trigger the 555.
The pad to be drilled is targeted by a laser with the beam guided at a shallow angle to the drill axis by a small mirror. This has a pwm supply to control the brightness.
I hope no-one is going to say "why don't you just use a microcontroller" - I wanted to test my circuit design skills!
So, the feed mechanism is the head positioning mech from a cdrom drive, I stripped out the optics and fastened the drill motor to the carriage with some steel wire. I fixed a slotted photo interrupter to the chassis so it detects the teeth of the final gear. The output of the phototransistor sits at just under 5v, so it's feeding the gate of a p-channel mosfet (Q1), which switches pretty well. Needed to put in a level shifting resistor to pull the input down a little. The mosfet feeds an npn so there is a place for other signals to feed in, and that feeds into the up/down control of a digi-pot.
There is a 1/2 556 monostable which generates a pulse to allow the digi-pot to change direction. It gets it's direction signal from flip-flop Q*. C11 creates a bit of lag to ensure the digi-pot's c/s input returns to 0v whilst the direction signal is still present.
When power is applied, Q4 holds flip-flop 1 in "set" until C12 is charged. This runs the motor until the mechanism reaches the top, where there will be a microswitch. This turns on Q3, fully charging C12 so the motor stops, and via 1/4 of the 4066 feeds pulses from the pwm into the digi-pot and winds it up to the top end. Then it sits and waits for a press on the "go" button (the plan is to use a footswitch for this).
When the button is pressed, it clocks a 1 into the 2nd flip-flop, sending the motor on it's way down to the lower set point. The comparator is wire-or'd with the output of the 1st flip-flop, so it turns off the motor before the 1st flip-flop gets it's clock input, which is delayed by C13 to allow for motor over-run. The 7474 has Schmidt trigger clock inputs so is happy with the slow rise (though the 4013 doesn't but is still OK). When the the clock signal arrives, the motor runs the drill back up until it reaches the upper set point. This resets the 1st flip-flop and stops the motor until the button is pressed.
The pulse to change over the digi-pot is obtained by detecting when the 2nd flip-flop changes state. Either C8 or C9 will always be charged via R23 or R25, connected to Q and Q*. When the outputs switch over, this is discharged through it's diode. The other cap then charges through it's resistor, slow enough to create a brief low pulse to trigger the 555.
The pad to be drilled is targeted by a laser with the beam guided at a shallow angle to the drill axis by a small mirror. This has a pwm supply to control the brightness.
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