It will be necessary to modify the main board by cutting the PC board track that connects U5 pin 3, and U6 pin 14. This will allow the ( new ) divide - by - 45 counter to be installed between the output of the NE555 clock generator and the input of the divide - by 16 counter. Wires must be attached to U5 pin 3, and U6 pin 14, and routed to the new divider board, to do this.

An additional wire must be installed on U6 pin 11, and routed to the new divider board, to provide a "reset" signal to the divide - by - 360 counter. Another wire must be installed on U9 pin 6, ( output of the calibration 1 - shot ) and routed to the input of the ANTI - JITTER circuit, on the COUNTER board.

The voltage regulator IC ( U27 ) normally runs very close to its "thermal overload" point, and needs a heatsink. No damage will result if it overheats, but nothing will work. I used a TO-220 heat sink, with ( approx. ) 3 square inches of surface area, and that seems to be adequate. The regulator is mounted at the edge of the PC board, so it may be possible to anchor it to the enclosure, which will serve as a heat sink. ( my enclosure was plastic, so I needed a separate heat sink )


It is also necessary to increase the frequency of the original NE555 oscillator to approximately 1/3 MHz. The exact frequency isn’t critical, but should not vary from 1/3 MHz by more than 20 or 25 percent... anything greater might require new component values in the LPF circuit, because the antenna rotation rate will also be changed, by an equal percentage. To achieve this, the 0.1 uF timing capacitor ( C19 ) must be reduced, and a "mod" must be installed.

Reason would dictate that the capacitor should be reduced by dividing its present value by 45, ( to 2200 pF ) which should cause the oscillator frequency to increase by a factor of 45. In fact, this did not prove true. Apparently, the basic 555 oscillator circuit is only capable of operating to around 300 KHz, and a "mod" was required, to achieve 360 KHz, as shown below :

I have now built about a half dozen of these, ( September 2001 ) and I find that the clock circuit ( shown in the schematic above ) is a "problem", and usually requires some extra attention to make it work properly. It seems that the 555 chip simply doesn't like to work "as advertised" at frequencies above 200 to 250 KHz, so the component values shown in the schematic must ( generally ) be adjusted to achieve proper operation.

In particular, the 555 malfunctions by generating irregular - shaped pulses in a series of "bursts", instead of a uniform series of identical pulses. This will not be obvious with a frequency counter, and can only be detected with a scope... a counter may indicate 360 "average" KHz, but the display will "jitter" wildly, because the clock frequency is unstable.

I have managed to make the circuit shown above work on all the units I have made, but I must usually replace the 270 ohm resistor with a 10K trimpot, and adjust it to achieve the desired result, ( observed on a scope ) then replace it with a fixed resistor, of equal value.

I have also found ( twice ) that the fall time of some 555 chips is too slow ( running at 360 KHz ) to provide reliable triggering of the divider chips on the COUNTER board. I added a 470 ohm resistor from pin 3 of the 555 to ground, and that fixed the problem.


In February 2003, I finally decided to try and eliminate the 555 from the clock circuit by replacing it with a PIC microchip, programmed to "simulate" a 555. I just got tired of dealing with the peculiarities of the 555 chip, when operated above 250 KHz. The results were good, so I decided to add it to the website, and I will probably use this chip in all future shipments of DF that require a 360 KHz clock. ( DFs with the RS232 or digital readout options )

The PIC chip I used is a 12C508. This is an 8-pin chip, like the 555. The power and ground pins for the 12C508 are the same as for a 555 chip, ( pins 1 and 8 ) but they are transposed. ( 555 power = 12C508 GND and vice versa ) To use the 12C508 chip it is therefore necessary to mount it on the backside of the board, or bend the leads 180 degrees so the chip is mounted on the topside of the board, but facing down.

The program for the PIC chip uses the internal ( precision ) 4 MHz oscillator as the time source, and generates an output signal with a fixed frequency very close to 360 KHz. You can get this chip from me for US$10 + shipping, or make your own by programming a ( blank ) 12C508 PIC chip. The source code for the chip can be downloaded by clicking [HERE.]

The DigiKey p/n for the chip is PIC12C508A-04/P-ND. There are actually several different versions of this chip that are suitable, ( different temperature or voltage range, etc. ) but this particular DigiKey p/n was the most abundant one they had in stock when I checked. ( Feb 2003 ) These are OTP devices, so it would be wise to get 2 or 3, in case of a problem.


It was mentioned in the original D/F that the antenna connected to the "090" input of the antenna board should reside on the passenger side of the vehicle, at the "3 o’clock" position. It was also mentioned that it is possible to connect this input to the antenna at the "9 o’clock" position, if the display LED’s were installed on the solder side of the display board, instead of the component side.

Unfortunately, there’s no "trick" way to do the same thing with the digital readouts.... in order for the digital reaout to work properly, the antenna input labeled "090" MUST be connected to the antenna installed at the "3 o’clock" position... if this antenna was previous wired to the "270" input instead, ( and the display LED’s were mounted on the solder side of the board, to achieve proper display rotation ) then the display LED’s must be removed and re-installed on the component side of the board, to make everything work properly. ( assuming the original display is still used )


This question also deserves consideration if you intend to use this readout with another type of Doppler DF unit... if the antenna array ( electrically ) "rotates" in a counter - clockwise direction, ( viewed from ABOVE the array ) then you have a problem... re-arranging the antenna wires will correct the problem for the digital readout, but it will ( accidentally ) "reverse" the direction for the original "LED circle" display.

Another way to "reverse" the direction of ( electrical ) rotation would be to invert ALL the data bits in the bearing counter of the original DF, using a 74LS04 inverter, or a similar chip... by inverting ALL the data bits, an "up-counter" ( 0,1,2,3, etc. ) becomes a "down-counter". ( 7,6,5,4, etc. ) If these inverters are installed DIRECTLY AT THE OUTPUTS of the original bearing counter chip, it will allow the original ( LED circle ) display to be used along with the digital readout... the calibration might need to be re-adjusted, but that should be the only "side effect".