The schematic is fairly simple and straightforward and uses a PIC16F628A microcontroller for measuring frequency and a high speed comparator for signal amplification and conditioning. Timer0 is used to count the input signal at pinRA4. In order to achieve the maximum possible resolution, the input signal is probed for 0.125 seconds and the prescaler value is computed accordingly. The input of the schematic is feed through a high speed comparator.
The two inputs of the comparator are set at about 1/2 of power supply voltage with 15-25mV difference between them so any AC signal with higher voltage will start switching the comparator. The input is protected with 1k resistor and two diodes limiting the voltage to ±0.7 V. The input impedance for low frequenciesis equal to R1 – 47k. For VHF range maybe it is good idea to replace it with 50Ohm value. The schematic have fairly low power consumption – with no input signal the supply current is 7-8mA and goes up to 20mA with 200+MHz input signal.
There are several digital clocks with analog displays around. They are somewhat unique giving the feel of the retro. With proper enclosure such clock becomes great design element. Following project was inspired by Alan Parekh multimeter clock. Originally there were three analog multimeters used in voltage mode to display hours, minutes and seconds. In this project there are three analog voltmeters used with their faces printed to match time scale – one for hours, one for minutes and third for seconds.
The clock code runs on PIC16F628A but time is synchronized every 30 seconds to master clock. Analog displays are driven through transistors by generating PWM signals that are equivalent to DC voltage. This part is fairly easy to implement on microcontroller. The clock also has a Chime circuit based on ISD1730 that gives “tick tock” seconds sound.
Using UV exposure technology is common way of building PCB at home. If you tried this, you probably know that exposure timing is very important to get precise image on photoresist. Overexposure or underexposure always lead to failure. So it is best to leave timing for timer. If you built your own UV box, then you should also build a timer.
Diyfan suggests simple PIC16F628A based timer project. It is really simple – 2×16 LCD displays time and status. Three push buttons allows to select time, start timer and stop timer. It also has speaker to play sound alarm when time runs out. Circuit simply send signal to a pin which can be connected to relay of your choice which then controls the lamps. Of course it can be used for other purposes including motor drive for some time, water sprinkler and other.
In most cases we use these HD44780 LCDs for displaying text messages. But also there are four bytes in LCD RAM that can store custom characters. This is handy when we need special symbol to be displayed. But here is a good example how text LCD can be turned in to graphical display and be used to chow pretty neat graphics.
Program library is written in PicBASIC and runs on PIC16F628A. Library works in similar way as you would write code for graphical display. You can set point coordinates, draw a line or circle. Library takes care of updating custom chars when needed and refresh LCD. As you can see refresh rate is impressive. Playing with refresh and pixel ON times there is also a doubled resolution mode.