Monday, April 19, 2010

Zilog Microcontroller General Purpose Frequency Counter with 7 segment display

This circuit is a frequency counter circuit original designed  to be used with a Geiger Counter.This circuit uses an 8 bit 8F0423 Zilog microcontroller to act as a frequency counter. The input to the circuit is driven through an optocoupler to isolate the source circuit. The output is displayed on a 3 digit seven segment display. There are 2 push buttons, one for reset and another to select between counter and frequency mode. Counter mode simply measures the counts whereas the frequency mode measures the counts per minute. The counter can accurately measure input up to 999,999, but only the 3 most significant digits are displayed. For example to display 19,832, 198 is displayed on the seven segment display and the x100 LED comes on. The overflow LED is used to indicate when the count has exceeded 999,999. It is fairly simple to add additional digits but for the purpose of my measurement the 3 most significant digits were sufficient. 

Technical Details:

The output from the optocoupler is connected to pin 11 of the microcontroller with a pull up resistor. A pull up resistor is used to ensure that there is a proper logic on the microcontroller input even when the output from the optocoupler is floating. An interrupt service routine (ISR) is triggered every time there is a transition on the port (from low logic to high logic and vice versa). The ISR keeps track of the count. Timer 1 is configured as a timer to indicate when a full minute is elapsed. This allows for the calculation of the number of transitions per minute. The 2 pins which say "To Geiger LED" take the input from any source.
Driving the seven segment display:
There are 7 LEDs per digit and to display 3 digits it will normally take 7x3 = 21 pins. This is beyond the packaging of most microcontrollers and a waste of resources. Instead a technique called multiplexing is used. Each digit on the display is turned on and off so rapidly that it gives an illusion that they are all on at the same time. This is due to the persistence of vision experienced by the eye. Timer 0 is configured to timeout every 1 ms and updates the seven segment display.
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Mode and reset buttons:
The mode and reset buttons are driven by their own ISRs to interrupt every time they are pushed down.  The internal pull up resistor is enabled on the pins connected to push button switches. This is to ensure there is always a clear logic level even when the push button is off (floating). 

Designed by Mindfront Technologies for  
The complete kit is available at

Monday, April 5, 2010

PIC 16F88 Microcontroller PIC based Tengu

Tengu derives its name from a mythical Japanese creature known for getting into mischief. Our Tengu, however is more earthly in nature.  It responds to voice and sounds and takes on different facial features depending on the intensity of the sound. If no sound is heard for some time, it changes from a happy face to a sad face and then goes to sleep. Gently blowing on his face wakes him back up to his usual happy self. 

The Technical Details:
The project is based on a Microchip PIC 16F88 which is part of their mid range of microcontrollers. The sound is amplified through a pre-amp circuit based on 2N3904 and fed to the RB7/AN6 (pin 13) of the pic microcontroller. The A/D converter on the microcontroller is used to convert the analog signal from the pre-amp. The LED Matrix is directly driven by the pic. In order to be able to display the entire smiley, the LED dot matrix is multiplexed in such a way that only one row out of the seven is active at any given time. However the rows are turned on and off so rapidly, the human eye sees it as a full picture. This effect is called the persistence of vision. In order to achieve multiplexing, timer 0 on the pic is running at approximately 1 ms timeouts to switch the row that is being displayed on the LED Matrix. Timer 1 is configured as a general purpose timer used to keep track of the time since any noise over the threshold was heard. This is used to change the smiley from a happy to a sad one. The code is written in C using MPLAB and Hi-Tech C compiler ver 9.80.

Parts List:
  • PIC Microchip 16F88
  • 7x5 LED Dot Matrix Display
  • 5 x 160 Ohm resistor
  • 2 x 10K Ohm resistor
  • 1 x 100K Ohm Resistor
  • 2N3904 NPN Transistor
  • Electret Mic
  • 1 x 0.1 uf capacitor
  • 1uf capacitor
  • 78L05 5v voltage regulator
Circuit Diagram 
Eagle Schematic available here

The gain I was getting on the amp circuit above was only moderate. I changed the circuit to use a LM386 based on Jose Pino's circuit and got a much better gain. The circuit diagram is below. The 10K potentiometer is used to adjust the sensitivity of the mic while the 100K potentiometer is used to adjust the gain. 

Designed by Mindfront Technologies for  
The complete kit is available at

Here is the video of the project in action:

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