You must have read all the circuits and explanation on the other pages at delabs to understand this, as i cannot repeat the same thing as i have done it more than once already.
The circuit is a 555 monostable, The push-switch is to trigger and generate a reset pulse for uC. The diode is for OR’ ing later. The High will go thru the diode but the Low of 555 cannot drink any current as diode blocks.
The bottom part of the Circuit is the CD4093 Schmitt nand based flipflop. See more at 555 Timer based Circuits
What is the Schmitt then ?, It is better to visit the links below to learn. It can make a sine or triangle or any shape waveform to square. It can help square a very messy waveform.
Two nand gates are connected to form a flip-flop toggle switch. When 555 gives out a pulse, the pulse is delayed by a R-C which results in a ramp, the third nand-Schmitt gives out delayed low trigger to toggle nand-flipflop-switch. The Nand-toggle-switch is rest at any time with the lower pushbutton. The fourth nand output gate is not really required. But the 4093 is quad nand, so unused nand inputs should not float , pull-up or pull-down. So it has just been added in circuit so that he can fit some role, or else he will be bored.
Now you can figure out how such blocks can be used like Lego blocks in your own amazing designs.
74HCU04 is a chip that was made for this purpose, HCT may not work for such a circuit. C1 and C2 can go to upto 33pF and R2 can be increased to make R2 * C2 = t.
Time constant much less than the period T of the crystal T = 1/F . This is to remove higher frequency components in the Oscillator.
More on Piezoelectric Crystal Oscillator
The circuit above is a parallel resonant oscillator circuit. The Crystal works by the piezoelectric principle, piezo means pressure. The electric field causes the impedance of the crystal to change. The LP Record Player needle is the reverse of this, the bumps on the spiral groove of the record applies pressure to needle which generates electricity. Both are piezo-electric effects.
When Instruments are designed a analog front end is essential and also as most equipment have digital or microcontroller interface the analog circuit needs to have digital access. The Circuits DACT0008 and DACT0009 are both useful in building instruments which have digital control.
Precision Attenuator with Digital Control
The Circuit DACT0008 is a programmable attenuator and the digital control can be a remote dip switch, a CMOS Logic Output like the A-B-C-D outputs of a decade counter, or an I/O port of a uC like 80C31.
The heart of the circuit is the popular OP07 OpAmp with Ultra Low Offset in the inverting configuration, 4052 a CMOS analog multiplexer switch enables the gain change, the innovation of the circuit is that the on resistance ( around 100 ohms) of 4052 switch is bypassed so that no error is introduced by its use.
The resistors used R1 to R6 can be 0.1% 50ppm if you will use a 3 ½ DPM i.e. + /- 1999 counts ( approx. 11 bit ), but for 4 ½ DPM ( approx. 14 bit ) you may need to have trimpots2 in place of R3, R4, R5 & R6 gain selection resistors to properly calibrate to required accuracy but for testing or trials use 1% 100ppm MFR resistors but the errors will be around 1%.
Output connect to DPM 7107/7135 or any other A/D Convertor or OpAmp Stage. Use a buffer at output if output has to be loaded by a value less than 1Meg. Use an inverting buffer if input leads have to have polarity where gnd is -In. See DACT0009 for details.
c. 4052 CMOS Switch
The 4052/51/53 Analog Multiplexers have an on Resistance of around 100E the highlight of the circuit is that the CMOS on resistance comes in series with the opamp output source resistance, which produces no error at output.
Digital Control Options
A and B can be controlled by I/O port of uC, like 80C31 so that the uC can Control gain. A and B can be given to Counters like 4029/4518 to scroll gain digitally. A and B can be connected to DIP switch or thumbwheel switch.