The BEAM Circuits Collection is a
"Chloroplast" solar engines
Craig Maynard's "baby" (and two
close relatives, courtesy of Wilf Rigter)
Craig Maynard dreamed up the Chloroplast solar engine (an
interesting design, tho' it uses two parts you probably
won't have sitting around in your parts drawer). Since I
really can't describe it better than Craig can, here's his
description (from the Chloroplast
web site), followed up by commentary and a related
by Wilf Rigter:
- Maintains a minimum voltage of around 5 Volts,
useful for maintaining digital logic or microprocessor
circuits [actually, the minimum voltage can be
either 3 or 5 volts, as a function of the parts you
select -- ed.]
- Has a hysteresis
from 5-7 Volts which is adjustable to suit the
designer's needs. This "bonus voltage" is discharged
across the motor, but discharging stops when the
minimum threshold voltage (around 5V) is reached
[again, minimum voltage is either 3 or 5 volts,
maximum voltage is limited by your solar cell(s)--
- Has more punch at a higher voltage than other
solar engines, making it suitable for larger,
- Uses only two three-pin devices and two resistors
available from most electronics retailers.
- Has no lock-up bugs.
- Very simple to free-form.
- Is very inexpensive.
- Extremely low quiescent current
( about 25 micro amps or less)
- Works the capacitor at closer to capacitor voltage
limits for greater efficiency.
Theory of Operation:
The heart of the Chloroplast is the Motorola
MC34164-3 Micro power Undervoltage Sensing Circuit (U1 in
the following diagram). In normal use, this component
monitors the voltage at pin 2, and applies a ground at
pin 1 ( out ) when the monitored voltage drops below 3V
(34164-3 monitors 3 volts, 34164-5 monitors 5 volts).
This low will then be used to assert a RESET on a
The output (an open collector) will be open when the
output is above the threshold voltage.
Through tinkering, I have discovered that placing a
220K to 270K resistor
in series with the input of U1 will produce a significant
in this sensor's output. For instance, a 220K resistor
as R2 will cause the output to ground until the solar
voltage hits 6.8V, at that point it will open (float)
until the solar voltage drops to 5.5V (a 1.3V hysteresis
). Different values of R2 will produce different
So how does this all work together? Well,
1.The solar panel will slowly charge up the
storage capacitor C1 towards 6.8V. U1 will assert a
ground (believing the voltage is too low) which keeps
U2 (a high gain darlington NPN transistor) open and
the motor OFF.
2.When 6.8V is reached, U1 will open. The base of
U2 will then be pulled high (through R1) and U2 will
turn on, allowing the solar energy in the capacitor to
discharge through the motor. The motor spins.
3.The motor will continue spinning and discharge
the capacitor until the solar voltage falls to
4.At 5.5V, U1 will assert a ground at its output
(Out), believing that the voltage is too low, and it
must apply a RESET. This ground turns off U2 and the
motor stops spinning and the system is ready for
Here's the schematic:
And if you want to free-form the circuit,
Wilf Rigter's take on the Chloroplast SE:
The MC34164 VS used in the Chloroplast SE is
different in one respect from the 1381
VS (used in the VTSE and
SEs) in that the MC34164 has an "open drain" output which
requires a pull up resistor
to the positive supply for the output to go positive.
The CSE uses a resistor
in series with the 34164 chip input and takes advantage
of the fact that the quiescent supply current
for the 34164 is higher when the output is low and
decreases to almost zero when the output is high. That
means the voltage drop across that series input resistor,
generated by the quiescent 34164 current,
must be added to the rated "turn on" voltage which raises
the voltage on the supply cap required to turn on the
34164 (ie to 7V). After the 34164 turns on, the quiscent
drops to zero and the voltage drop across the series
also drops to zero, so that the voltage at the 34164
input pin suddenly jumps up to the supply cap voltage.
The voltage at the "open" 34164 output pin is now pulled
up to the supply through the output resistor,
but the output pin voltage only rises to the forward
voltage of the darlington transistor base emitter
junction or about 1.2V. The darlington turns on and the
collector supplies current
to the rest of the circuit.
The supply cap voltage starts to drop as the motor load
etc depletes the cap charge until the supply voltage
reaches the 34164 rated turn off voltage (ie 5V) and the
In one of my earlier MC34164 hysteresis
SE designs (hySE), the 34164 upper threshold was raised
by inserting a resistor
in the ground lead and using the voltage drop generated
from quiesent current
and sinking the current
from the output pull up resistor.
Craig Maynard greatly improved on that design using the
same principle but simplifying the circuit
in his elegant Chloroplast design.
One popular misconception about these the Chloroplast
SE is that it provides a convenient regulated 5V supply
for powering up micros etc when in fact all it provides
is a fluctuating supply voltage whose lower level can be
set to 5V minimum. This is just a matter of design choice
since all but the simplest latching or timed SEs offer
More recently, Wilf posted two Chloroplast-related SE
designs ("Miller style"
Chloroplasts) -- here are their schematics, and Wilf's
Here is a yet another hybrid SE, this time
combining the principle of the Miller
Engine with the Chloroplast SE. Two versions are
shown with the top one using a Darlington transistor
and the bottom using 2 separate NPN transistors
which have a lower "on" voltage and deliver slightly more
power to the motor. Both MS Chloroplast SE trigger at
4.8V and reset after a time determined by C2. The value
of the main storage cap
C1 can be 10,000uF to 1F and the value of timing C2 can
be 1uF to 100uF.
This SE design can also be used with the 3V MC34164-3
chip, 3.4V solar cells and 2.5V super caps for a lower
voltage applications (2.8V).
My prototype of the lower schematic sits in the window
near the 49th parallel and with C1=10,000uF and C2=100uF,
the trigger point is 4.8V and the cycle time was about 20
sec with yesterday's overcast sky and about 2.5 seconds
in this morning's glorious sunshine. The efficient
mightymo gearmotor turns about 4 rotations in 1
For more information...
The Chloroplast Solar Engine homepage is here
(3 images, 78 kBytes inline); Wilf's post of his
new Chloroplast variants is here.