LED Frequency Counter

LED Frequency Counter Circuit Details

LED Frequency Counter Connections for
the Salmon Pink Colored Boards Dated 10-1-2000

Construction of the PCB
Connecting to the Receiver || Using the Frequency Counter
LED Frequency Counter Parts List

Construction of the PCB

Soldering Tips || To Cut or Not to Cut the Board
Construction of the PCB
Fixing Ground Plane Error
Step 1 - Resistors || Step 2 - Diodes || Step 3 - Capacitors
Step 4 - 2N3906 Transistors || Step 5 - 7805 and 10MHz Oscillator || Step 6 - PIC IC socket 28 pin
Step 7 - Solder 5 ICs || Step 8 - Solder two .01 caps || Step 9 - Solder the 7 LED displays
Step 10 - Solder the 220 ohm resistors || Step 11 - Insert the PIC16F72 into the Socket
Step 12 - Testing the Counter

Soldering Tips

The best soldering tip is use is a very narrow one. Check this page for pictures of some tips. A temperature regulated iron is the best to use.

Plated Through Holes

This PCB uses plated through holes. It is very difficult to remove parts from plated through holes unless you can remove one lead at a time. This means double checking all multiple pin parts (ICs, oscillator, voltage regulator, transistors) BEFORE soldering!

If you solder a part and realize it is in wrong, you have to cut all the pins off the part to get it out, which means getting a new replacement. Email if you do this and I will help with a new part.

Fitting/Soldering The ICs

The ICs have their pins flared out slightly. They will not fit into the board without a little adjustment. The pins need to be gently pushed in so they are straight down. This can be done with your fingers on each side of the IC pushing inward toward the middle of the IC. Go slow, check until they fit.

Hold the IC against the board with one finger, get some solder on the solder tip, then tack on one lead of the IC to the board. Next, set the board down and solder another pin to the board feeding solder to the tip and pin and get a good solder joint.

With your finger against the top of the part holding it against the board, reheat both soldered pins to get the IC firmly against the board. Finally, solder the rest of the pins.

Large Soldering Tips

When using a solder tip larger than what is recommended, the best way to make it work is to start by placing the solder tip ABOVE the hole on the lead, get solder to flow on the lead adding some solder to the soldering tip and lead. Then move the tip down to the hole, keeping the tip against the lead, add a little more solder. The solder should be seen going into hole.

If you have a small round blob on top of the hole, try again, and check the top of the board to see if the solder has flowed through the hole.

To Cut or Not to Cut the Board

The board can be cut into two boards through the rows of holes in the middle of the PCB. See below.

Tin snips work well for cutting the board. It is recommended to start cutting as shown on the right side, as shown above, (when the board is populated) to prevent damaging the 6.8K resistor near the holes on the right side.

Cutting the board now, before populating the board, is a lot easier. However, you can change your mind later.

If tin snips are not available, a straight edge (i.e., a ruler) and a utility knife can be used to score both sides of the board through the holes. Go as deeply as you have patience for, then break apart.

If you are planning to mount the frequency counter between Board 1 and the bottom unetched PCB the boards need to be cut.

If you plan on building a case for the receiver and mounting the frequency counter on the front face, then you may not need to cut the boards depending on your design. Also, other projects may not require the boards to be cut.

Construction of the PCB

LED Frequency Counter Parts List

Fixing Ground Plane Error

There is a break in the ground plane on the right side of the board (viewed on bottom side). This break will be fixed on most of the boards sent out, but you must double check this first and fix it if not previously done.

If fix is done, click here to skip to next step.

This ground plane break will cause problems when the boards are cut across the holes. On the bottom of the picture above you will see "Ground Connection for 4077 pins". If this hole is cut so that the copper does not connect the top and bottom grounds, the 4077 will lack a ground connection.

Fixing the ground plane break on the right side of the board will fix the ground for the 4077 IC. If the 4077 IC is not grounded the counter will be locked with a blank readout.

Closeup picture of the ground plane brake.

The fix is simple. Scrape the soldermask from the ground plane just above and below the break with a sharp knife, razor blade, or 220 grit sandpaper. Then solder a wire across the break. See pictures below:

A closeup of the soldermask removed with 220 grit sandpaper.
Easier to do than the picture represents.

Tin the exposed ground plane first, then a wire
(or piece cut off from resistor or capacitor)
is soldered across the gap.

Step 1 - Resistors

The footprints for the resistors have the value printed inside the footprint. Double check the value when inserting a resistor. Bend the leads of the 1/8 watt resistors about 1/16" from the body of the resistor to fit into the footprint. The leads of the 1/4 watt resistors are bent right against the body of the resistor.

The 220 ohm resistors will not be inserted in this step.

1 - 10K, 1/8 watt, bend leads 1/16" from body

3 - 1K, 1/8 watt, bend leads 1/16" from body

4 - 100K, 1/8 watt, bend leads 1/16" from body

Clean the leads on these resistors if they are dirty/black.

2 - 6.8K, 1/4 watt

Step 2 - Diodes

4 - 1N4148 diodes, bend leads 1/16" from body

Make sure the black band on the diode is inserted the correct way. The wide white band on the footprint is where the black band should go on the diode. To double check, the bands should be opposite each other at each pair.

Step 3 - Capacitors

1 - .1
1 - 2.2mfd electrolytic
1 - 100mfd electrolytic

Notice the electrolytics are polarized, they must be put in with the ground and positive side correctly. The short lead on the electrolytics is ground and the long lead is positive. The plus "+" side is marked on the board. Put them in incorrectly and they could explode or get very hot!

8 - .01

Start on the left side of the board and work around the top edge to the left side to get them all.

2 - .01 bottom side of board - Do not solder now!

This picture shows the location on the top of the board where two .01s are soldered underneath the board.

These will be put on the board in Step 8 after the ICs and 10 MHz oscillator have been soldered on the board.

Step 4 - 2N3906 Transistors

All kits shipped after 3/1/09 have 7 - 2N3904s with straight leads for the LED cathode drivers and one flared (on small cardboard) for the 8th one. Click here to skip modifying the flared transistors.

If you get flared leads on all 8 of the transistors, see picture below, straighten the leads of seven transistors before inserting into the board.

___First - Cut transistor from cardboard.
___Second - Bend center lead backwards toward rounded side of transistor.
___Third - Straighten outside leads with a needle-nosed pliers.
___Fourth - Bend center lead straight down with slight offset as shown in picture.

Transistors which have straight leads need a slight offset for the center lead as shown on the far right transistor in the picture above.

It is not necessary to push the transistors against the PCB. The footprint for the transistors was necessary because of the tight fit between the traces for the LED readout and that meant a slight offset for the center lead was needed.

7 - 2N3906 straight leads

Insert 3 2N3906 in the top row. Notice the flat side
of the transistor and match it to the footprint flat.

Solder the lower row of pins first. Then clip those leads so there is better access to the top row of leads. Solder the remaining leads.

Insert 4 2N3904 in the lower row. Match the flat of the transistor to the flat of the footprint.

Solder the lower row of pins first. Then clip those leads so there is better access to the top row of leads. Solder the remaining leads.

Check for solder bridges and bent wires shorting against adjacent pins after soldering and clipping all the transistor leads. Solder bridges can be easily removed by wiping the tip of the soldering gun between the pins.

1 - 2N3906 flared leads

Cut the transistor from the cardboard and insert in the place indicated above by the yellow arrow.

Note: This step was overlooked in the first run of the instructions, so that is why you do not see this 2N3906 on the board in the following pictures.

Step 5 - 7805 and 10MHz Oscillator

Location of parts. Please note the dots in the lower left hand corner of the 10MHz oscillator and the front and heatsink of the 7805 voltage regulator.

Be careful when inserting these parts, they must have correct orientation!

The 10 MHz oscillator can must have the dot in one corner at the lower left hand side as shown above in the picture. The dot is in the footprint and just outside so you can double check after the part is in place.

The 7805 voltage regulator must have the front of the part toward the outside of the board. Also, the front side of the part should be pointed to the part number "7805". The metal heat sink for the 7805 is the back of the part and should be toward the center of the board. See picture above.

Step 6 - PIC IC socket 28 pin

The orientation of this socket is important! Notice the notch on the right hand side of the socket.

The notch on the footprint and the notch on the socket should be seen together as shown in the picture.

Solder all the pins as shown in the picture. The arrows point to the beginning of both rows.

The one on the bottom can easily be overlooked as there are other holes very close by.

Step 7 - Solder 5 ICs

The 5 ICs that are soldered in this step. The most important part of this step is getting the notches correctly placed as shown on the footprints on the board.

There are 2 ICs that require special attention.

The first one is the 74AC151:

Pin 5 on the 74AC151 has to be cut off. Pull pin out from the IC as shown above and cut it off. Pay particular attention to the location of the notch on the IC.

The picture above shows the location of the IC and the location where the pin is cut for. After the pin is cut insert into the board and solder.

The second one is the 4077:

It was a surprise to see this dot on the part at the wrong location! Insert this part according to the notch only! Ignore the dot.

Next, insert and solder the 2 74HC4046 and the 4066:

Be sure you have it right BEFORE you solder. Removing parts from plated through holes is very difficult.

Step 8 - Solder two .01 caps

This picture shows the location on the top of the board where two .01s are soldered underneath the board. At this time they will be covered up by the oscillator and 28 pin socket.

The first step is to fill the holes with solder.

The two .01 caps are soldered to the holes just filled with solder. Cut the leads of the capacitors about 1/8" and pre-tin to make it easier to solder to the solder filled holes.

Step 9 - Solder the 7 LED displays

Look closely at the pin pattern of the readouts and it is obvious that the readouts will fit only one way. The notch is at the top of the readout and the decimal point is at the bottom right side.

The LED displays all soldered.

Step 10 - Solder the 220 ohm resistors

One Board

If the board is left as one piece, the 220 ohm resistors are soldered in as shown above. These resistors limit the current to the segments of the displays. Note the a through g, and dp (decimal point) lettering, at the bottom of the resistors.

Cut Board - Two Boards

If the board is cut at the row of holes, the 220 ohm resistors are used as spacers between the two boards and soldered. Then the cut leads are used to make connections between the other 8 connection holes (1 through 7 display drivers and ground).

Start by inserting the resistors in the pic board. Bending over the leads as shown above will hold them in place. See below the spacing between the resistors and the board. Clip the leads after soldering.

Leaving a 1/8" space will allow the boards to be bent at any angle for mounting.

The boards fit Back-to-Back. Insert the resistor leads into the back of the LED board. The picture shows another 1/8" spacing, but this is optional depending on how you want to mount the board.

If you are going to mount them parallel, then no spacing is required, with the pic board bent down from the LED board (to provide maximum spacing from Board 1 of the receiver to minimize interference), then another 1/8" spacing is good.

Top of LED board showing resistors leads coming through. After soldering, clip the lead short. Keep the cut leads.

Straighten the boards parallel to each other and put the clipped leads in the rest of the holes, the one next to C is the ground lead, 1 through 7 are the display driver leads.

MSD means Most Significant Display, LSD means Least Significant Display.

Picture showing the leads coming through the LED board. Solder and clip. Be careful not to have a lead touching its neighbor when soldering. Bend the leads as necessary.

Step 11 - Insert the PIC16F72 into the Socket

Remember to bend the pins toward the center of the IC until they are pointing straight down. Look down the rows of pins to make sure one is not bent further toward the center than the others. If one is bent too far, it may bend against itself or against the bottom of the IC, not going into the socket correctly. This will cause an intermittent error when the counter is turned on.

Lay one side of the pins into the socket, do not push down.

Roll the other side of pins into the socket. They may not fit in. If they don't, push on the pins slightly with your finger until they slip into the socket. Or, bend them some more toward the center of the IC and try again. Be careful, go slowly.

Now push down on the IC to set into the socket. Look down each row of pins to make sure, as best as can be seen, that all the pins are inserted into the socket.

If you do bend a pin, either up onto itself, or up against the IC, carefully straighten the pin. Reinsert the IC watching the previously bent pin to make sure it goes into the socket correctly. Using needle nose pliers makes straightening a pin easy.

You can get away with one rebending of a pin for sure, a second rebending of the same pin gets a little iffy, three, maybe not.

Step 12 - Testing the Counter

The "OSC IN" box is shorted to ground with a piece of resistor clippings as shown in the picture.

A couple of clippings are used to make connections for the "VFO In" and "GND". A couple more clippings are used to make connections for "12 Volts" and "Gnd" connections. The clippings are made into a tight U shape, inserted from the back side of the PIC board, twisted together, and soldered.

Apply a signal from a signal generator or the receiver VFO to the VFO IN and GND connections. While watching the display, apply 12 Volts to the 12 Volt and GND connections.

The first display to appear will be a single dot as shown above at the third digit from the right. The counter is reading the OSC IN signal.

About a second later, the OSC IN signal is displayed, which is 0.00 Hz (sometimes 0.01 Hz).

A second or two later, the counter now reads the VFO signal as shown in the picture.

If your counter follows the sequence above, then it is in proper working order.

The counter fixed this way can be used as a general purpose frequency counter for any purpose.

Disconnect the short across the OSC IN and Gnd connection before continuing to "Connecting to the Receiver".

Connecting to the Receiver


Send E-Mail || Amateur Radio Receivers || Electroluminescent Receiver || Back to Ham Radio Instructions
Back to LED Frequency Circuit Details

Last Update: 11/22/2021
Web Author: David White, WN5Y Free Website Translator