Power Supply .
Because of the low current requirement of most my designs, I usually prefer 9 volts as a convenient voltage supply source and for that reason the frequency counter has also been peaked for that voltage .
Below is the schematic for the power supply used as bench instrument and the parts list required . For stability and precision it is important that the power source be regulated and I have used a 9 volts regulator for a 12 volts power transformer source . The system will work well with a higher voltage supply with the following considerations ;
The supply voltage must be at least 2 volts higher than the regulator voltage when under full load .
The current limiting resistors value of R9 to R22 must be increased to limit the segments LED current within the 10 mA range .
A battery pack can be used but again a regulator should be used so that the system does not suffer degradation due to battery voltage drop . A battery voltage monitor as described in the Digital Capacitance Meter project can used to monitor the battery voltage drop and set to about 1.5 volts above the regulator voltage as an indicator of low battery voltage .
As an option a wall transformer module can be used . Most wall transformer units are NOT well filtered and care must be taken to identify the voltage polarity of the output connection plug . Whether you use an AC or DC wall unit it is strongly recommended that the portion for the power supply circuit after the transformer , that is from the rectifier bridge and after be used to insure of the right voltage polarity and filtering .
Power Supply Parts List
Select an enclosure large enough to accommodate the circuit and the transformer . I had a spare plastic box the size of which was perfect measuring 5"W x 3"H x 6"L .
I obtained the transformer from an old digital alarm clock from Sally Ann , the older the better as they all have a small 18 to 24 volts center tap transformer at about 300 mA .
A box with a plastic face panel is best as it is easy to cut a window to accommodate the six digit read-out covered with a thin red plastic as a filter as seen on my finished project , only the switch and two input plugs remain to be installed as shown . I used dry lettering transfers for the text covered with a light sprayed coat of clear lacker to protect the lettering from erasure .
Other than the switch ON/OFF with range selection there is no calibration required . A first check can be made by using a low voltage transformer with the primary connected to the AC line and measuring the 60Hz output on the low range of the Frequency Counter.
I almost forgot to mention the and LED is used as a power on indicator , as shown on the schematic a 1K resistor is used in series and connected from the positive bus to ground . ( not included on the parts list )
An RPM Meter
From the basic Frequency Counter project presented , George Vasiliou of Greece has developed a precise RPM counter using a magnetic sensor to detect teeth on a wheel and a formula to calculate and calibrate for any size wheel/RPM . For any additional information George can be reached at "firstname.lastname@example.org "
And please do not use the output from the wall AC outlet to test your meter .
Introduction construction-1 Construction-2 Construction-3