None of the three headers are
absolutely necessary. You should
solder the leads from the sensor
directly into the holes where H1
would be installed since the box is
not tall enough to accommodate
the mating housing.
If you are not going to
program the PIC on the PCB, then
H3 is not necessary. If you do
want to use the headers, the part
numbers and quantities in the
Parts List have enough pins for all three headers. You
simply need to cut them to size.
Although a crimper makes the job of assembling the
pins for the housings easier, it is not required. Before I got
a crimper, I used a pair of small needlenose pliers to crimp
the wires to the pins. It took a little practice (with several
failures), but I did learn how to do it.
Take a look at Figure 3 for a sample of what the
housings and pins look like.
The op-amp used in the circuit
is in an SOT- 23-5 package. The
spacing between pins 1, 2, and 3
is 0.95 mm (0.037 in). I highly
recommend that you work using a
lighted magnifying glass and a very
fine soldering iron tip and fine
solder. My iron has a 0.8 mm tip
and I use 0.015 in diameter solder.
The spacing on the other
semiconductors is not nearly so
I recommend a good pair of
tweezers to hold the parts in place
while soldering. I generally apply
solder to one pad on the PCB and
then place the component while
heating that same pad again. Then
solder the remaining pad(s).
There is a sizable pad on the bottom of the board for
the negative surface of the battery — you should apply a
smooth covering of solder to ensure good contact.
Although there is a mounting hole in the PCB for a 4-
40 bolt, I leave the PCB floating and use a rubber band to
hold the lid onto the case.
Note that the buzzer is polarized. The PCB silk screen
shows which terminal is the positive. Also, you will need
to bend the leads in
order to surface-mount
I filed a notch in
the lid for the sensor
wires and drilled a hole
in the lid to allow more
is no protection against
Please be sure to insert
the battery with the
negative side against
I hope you find
this circuit as helpful as
I do. NV
34 October 2016
■ FIGURE 3. Close up of a sample header
which I use for programming my PICs.
Ref/Des Value Name Qty $/ea Total
B1 BR2032 P189-ND 1 $0.28 $0.28
BOM1 Battery Holder BAT-HLD-001-ND 1 $0.28 $0.28
BOM2 Box 377-1692-ND 1 $0.45 $0.45
BOM3 Box Lid 377-1693-ND 1 $0.37 $0.37
BOM4 Circuit Board see text 1 $2.10 $2.10
BOM5 Housing for Headers WM5341-ND 1 $0.60 $0.60
BOM6 Pins for Housing WM2562CT-ND 10 $0.11 $1.14
BOM7 Probe Pins 609-1474-ND 2 $0.10 $0.20
BZ1 Buzzer 668-1097-ND 1 $1.13 $1.13
C1 0.1 μf 311-1335-1-ND 1 $0.09 $0.09
C2 1 μf 1276-1119-1-ND 1 $0.18 $0.18
H1, H2, H3 S1012EC-20-ND 1 $0.36 $0.36
Q1 2N7002 568-5818-1-ND 1 $0.14 $0.14
R1, R3, R4 1M 311-1.00MCRCT-ND 10 $0.02 $0.21
R2 3M 311-3.01MCRCT-ND 1 $0.10 $0.10
R5 39 311-39.0CRCT-ND 1 $0.10 $0.10
R6 100K 311-100KCRCT-ND 1 $0.10 $0.10
U1 MCP6421 MCP6421T-E/OTCT-ND 1 $0.53 $0.53
U2 PIC12F1572 PIC12F1572-I/SN-ND 1 $0.60$0.60
Total cost $8.96
These parts are only necessary if you are going to use the headers and mating housings.
The listed part numbers, and quantities, are adequate for all three headers.
Buying 10 is cheaper then buying three. Prices are subject to change without notice.