This is the ultimate circuit for a Photopopper. It
is also phototropic due to the light-detecting nature of the flashing
It converts the voltage
from any number of solar cells to drive motors up to 12v. The minimum number of
cells is two but four is probably the most practical minimum.
Almost all the Photopopper circuits described to date use a 1.5v pager motor
and a 3.5v to 5.5v solar panel.
But for the majority of constructors, these components are not available near
where they live or even a "phone call away."
However the two items that are closest to the requirements of a solar panel and pager
motor are individual solar cells and a 1.5v or 3v hobby motor. I know these
motors aren't as efficient as pager motors but if you need to use readily
available components, this is what you have to get.
Some motors from old VCR's etc are 12v types and these can be
used with this circuit.
USING WHAT'S AVAILABLE
Individual solar cells produce 0.45v and come in output current capabilities of 100mA,
200mA and 500mA.
Creating a panel from individual cells will be larger than the mini solar panels
on the market but when you see the current capability of some solar panels,
you will see why individual cells are a viable alternative. Small solar panels
deliver 7mA, 15mA, or 30mA @ 3.5v to 5.5v The cost of the highest output type
is twice the cost of our design.
When you use this circuit with individual cells and a salvaged motor, you will be able to create
sun powered vehicles at a
lower cost to anything else. A single motor design such as "Solar
Roller" can be designed using the circuit below or a two-motor design
using two Solar Engine sections. We have called our single motor design Sun
Roller MkIII and its constructions starts on page 11.
Our two motor design , similar to the Photopoppers described on previous pages
is called Robo Roller MkII and its construction starts on page 12.
With our circuits and components you will be able to get things up-and-running sooner.
Firstly, let's look at the circuit.
Only one Solar Engine section has been
How much of
the Robo Roller Circuit can you remember? Click above and try this
The circuit consists of two building blocks. The Photopopper circuit designed
by Ben Hitchcock and a voltage multiplying (or voltage increasing) circuit from
a Solar Charger project we will be presenting in a future issue.
We have already covered the Photopopper circuit and
the items we have added are green LEDs to increase the "turn-on" level. Each LED
(including the flashing LED!) increases the "turn-on" voltage by
approx 2v. This gives a wide range of voltage for the type of
motor you intend to use. The circuit also includes a 220R to limit the turn-on
current to the base of the Darlington transistor. This is very important as it represents wasted
The Solar Charger circuit is very simple in design but much more complex
in discussion as it employs a transformer. Although the transformer consists of only two windings, the way it works could
take a page to explain fully.
It is a self-oscillating building block called a Blocking Oscillator.
The transistor in a Blocking Oscillator draws current only during a small portion of
the cycle and the output is a pulse. (Other transformer circuits looking very
similar to this one are sine-wave oscillators. A sine-wave oscillator
does not generate a high voltage across the primary winding.)
At switch-on, the base receives current via the 15t winding and 1R resistor and this turns the
transistor on a small amount.
Current flows through the collector-emitter terminals and
through the main winding (this can also be called the Primary).
This causes magnetic flux to be produced and the flux cuts the turns of the 15t
feedback winding and the transistor is turned on more. The effect is called
re-generation or positive feedback and the transistor keeps turning on until it
is fully turned on.
At this point maximum current is flowing through the primary and
maximum flux is produced in the core of the transformer but this flux is not expanding
flux and thus it does not produce a voltage in the feedback winding. The
current into the base of the transistor reduces slightly (there is still
current flowing via the 1R resistor) and the transistor turns off slightly. The current in the primary winding reduces and
this causes the magnetic flux to reduce. This is called collapsing flux and it
causes a reverse voltage to be produced in both the main winding and feedback winding.
The reverse voltage has the effect of reducing the base voltage and also the
base current and causes the transistor to turn off even more. This keeps
happening until the transistor is completely turned off.
The action occurs very rapidly and here's where the clever part comes
When magnetic flux collapses very quickly, the voltage produced in a winding is
quite high. It can be 10-times or even 100-times greater than the applied
voltage and this is how we get the value of "Q" for the transformer. "Q"
is a term for efficiency (or multiplication-factor) and in our design we get output spikes exceeding
However don't panic. Nothing will be damaged. The Photopopper section
removes the charge from the electrolytic before it has time to go higher than
Why does this circuit work with
any number of cells and any voltage motor?
There are two different types of circuits in electronics. One type is called "Power
Electronics" and the other is called "loose electronics."
Power electronics is where you have sufficient current behind a waveform for it
to damage the surrounding components.
Loose electronics is where a circuit adapts to the "mis-match" of
the surrounding components and nothing is damaged.
The Solar Charger section is an example of Loose electronics. It will adapt to almost
any input voltage and accept any load, without being damaged. This circuit will
operate on 4 to 8 cells and drive a motor from 3v to 12v.
The circuit produces high voltage spikes but they are very narrow
and do not contain much energy. The large reservoir electrolytic
accepts these spikes and stores them. The result is the voltage across the
increases a small amount on each pulse.
The circuit operates at more than 20,000 cycles per second and is
effectively an OSCILLATOR. The actual frequency of operation is
determined by the amount of ferrite in the core of the transformer, the applied
voltage, the load on the circuit, the number of turns on the feedback winding
and the type of transistor. It cannot be readily altered.
Converting a low voltage to a high voltage is not very easy and some losses
naturally occur. Our circuit is about 30% efficient but if more cells are
added, the efficiency increases.
WINDING THE TRANSFORMER
The core of the transformer is an old 10mH choke. This provides an ideal bobbin
for winding the transformer.
The first thing to do is remove the wire from the choke. You will not need it.
Both windings are wound with 0.25mm enamelled wire. (0.25mm = 0.01" = #30AWG = #30B&S)
Mark the start of the wire with "white-out" and solder it to one of
wires connected to the core. Wind 15 turns on the core and solder the end to
the other wire connected to the core.
The main winding consists of 100 turns over the feedback winding. This can be
"jumble-wound. It does not matter how it is wound. Mark the start of
the winding and add the 100 turns. Twist the ends together. Leave about 3" (6cm)
for connecting to the circuit.
CONSTRUCTING THE CIRCUIT
Construction notes for the circuit are on page
Points worth noting:
You cannot measure the output current of a single solar cell with an
ordinary multimeter and get an accurate short-circuit capability. This is because the
output voltage is very low and the impedance of the multimeter will not
allow the full current to flow. Even 4 cells will not provide an accurate
The circuit for the Solar Charger has a very low impedance to cater for the
low supply voltage. It will operate down to 1v but the efficiency
increases considerably as more cells are added.
The ZTX851 transistor we have suggested has a very low collector-emitter
voltage (0.1v) when switched ON and this improves the output considerably.
If you use any other type of transistor the output will be 20% less.
If you build the Solar Charger circuit and need
to test it, you must place a large electrolytic on the output to
store the pulses of energy. You cannot measure the output with a
multimeter with the electrolytic because the pulses are very brief and the meter will not
If you need to measure the current capability of the circuit, you need to
place a 100u electro across the output and then fit your ammeter between the
(filtered) positive rail and a 2200u.
When winding the transformer you must remember which end is the start of
each winding. You can either scrape a large amount of enamel from the start
end or apply "white-out."
If the windings are connected to the rest of the parts incorrectly, the circuit will not oscillate.
Simply reverse one of the windings (not both).
Both windings must be wound around the same way on the core of the
transformer for the "start of winding" to apply. Either
remember all the requirements or wind the transformer and reverse one
winding if it doesn't work.
You can also use 4 red or green
LEDs to provide the reference voltage and this will work the same as the
previous circuit except when the light intensity is low. It may tend to
"lock-up" as the charging current equals the loss through the
LEDs and the circuit will fail to "fire." The inclusion of the
flashing LED prevents this lock-up.
The circuit may also tend to "lock-up" at high solar levels due
to the charging current being sufficient to keep the motor turning due to
the high output of the solar cells.
This is also due to the voltage across the motor keeping the PNP transistor
turned ON. The solution is to provide slight resistance for
the motor shaft so that "back emf" is not produced.
For Robot parts at very low prices see: Andy's Robot Parts