volts to operate, so for the battery power I
decided to use two 18650 lithium-ion cells.
I use these in my LED flashlights, and a
pair of cells with a charger costs less than
$10 — as long as you’re willing to wait for
delivery from China.
I found a low drop-out linear voltage regulator (part
BA05CC0T) that delivers five volts from the minimum
charge level of the batteries. This part does require at least
a 1 µF bypass capacitor on the output side for it to work
properly. As you’ll see shortly, this value was increased
significantly after testing.
Other components for the Deluxe Servo Tester include
a Microchip PIC16F1825 14-pin processor, various
connectors, and a 3D printed case. (Check out the Parts
List with the sources.) Figure 2 shows the 4D Systems
display kit I used.
This has a 2. 4 inch diagonal display area. While this
may seem small at first, you’d be surprised how many
controls and messages you can squeeze into that amount
of space. There are multiple versions for each 4D Systems
display, ranging from bare-bones kits which have just the
display, to special kits for interfacing with Arduino or
Raspberry Pi systems. I usually get the developer kit which
includes a cable, microSD card, and programming adapter
in addition to the display.
Overall, the circuit is quite simple. The display is
connected to the processor serial port, and a single I/O pin
from the processor sends the control signal to the servo.
Additional processor pins are used to check the battery
voltage, provide debugging information, and light the
heartbeat LED. The circuit is shown in
Ideally, any system that uses lithium-ion batteries should have some protection
against over-discharge. Some batteries
have built-in protection, but I’m not sure
the ones I have include that. To address
this, I added a voltage display on the
The PIC processor accepts an analog
input and can convert a voltage between
zero and five volts to a number between
0 and 1023. The combined operating
4D Systems 24PTU Display Kit 971-SK-24PTU
PIC16F1825 Processor 579-PIC16F1825-I/P
Positive battery contact ( 2) 789-ACC-AAPOS-5223
Negative battery contact ( 2) 789-ACC-AANEG-5201
5 volt LDO Regulator 755-BA05CC0T
Prototyping Board Adafruit #1609 www.adafruit.com
18650 Battery ( 2) & charger eBay
Slide switch and other parts Parts on hand
Linear actuators www.firgelli.com
CCS PIC C Compiler www.ccsinfo.com
4D Systems documentation and software www.4dsystems.com.au
Author’s 3D printer information www.instructables.com/id/
Author website www.jgscraft.com
32 SERVO 12.2015
Figure 2. Display kit.
Figure 3. DST circuit.