Project 03: LED Flasher, Voltage Boost

Tags

LED, flasher, blinker, pulser, oscillator, voltage boost, voltage doubler, switched capacitor, DC-DC converter.

Difficulty Rating

3 on my scale.

Purpose

This is an expanded version of our starting circuit, and with a special capability: it runs on one less battery. It's an LED flasher that's powered from just one AA cell. It can also be used to flash a blue or white LED with just two AA cells.

Bill of Materials

These parts are all in the basic kit, which I described earlier.

Label	Description	Image
T1	NPN Transistor, 2N3904 or similar.
T2	PNP Transistor, 2N3906 or similar.
R1	Resistor, 2.7 megohms (= 2,700,000 ohms). Color stripes: red-purple-green.
R2	Resistor, 910 ohms. Color stripes: white-brown-brown.
R3	Resistor, 1 kilohm (1000 ohms). Color stripes: brown-black-red.
R4	Resistor, 10 kilohms (10,000 ohms). Color stripes: brown-black-orange.
R5	Resistor, 68 ohms. Color stripes: blue-gray-black.
C1	Capacitor, 220 nF (220 nanofarads = 0.22 microfarad).
C2	Capacitor, 220 μF (220 microfarads).
D1	Diode, 1N5819 or similar
LED1	LED. Red-emitting (yellow, orange, or green also OK).

plus these general parts...

Solderless plug-in breadboard
AA carbon-zinc cell

Assembly

Click on the image to see a larger version.

Start by inserting the two transistors as shown. Note that their 'faces' (flat sides) are shown upward. T1, the NPN type, I've painted white to be visually very different than the PNP. (You should paint a dot on all your NPN's so you won't mix them up with your PNP's).

Next, insert LED1, R5, and R3. LED1 is polarized — the lead nearest the flattened edge goes in towards the right.

Then insert C2, but see that its positive (+) lead goes in to the left, and its negative (–) lead on the right. C2 is a polarized capacitor. It has a black jacket with a white stripe next to the negative (–) lead.

Next, install R4, C1, R2, and R1.

Then we get to D1. D1 is a diode, so it is also polarized. The lead that's nearest to the white band goes in towards the left.

Attach the power leads only as the last step. Before you power-up a circuit, always double-check that the parts are connected correctly. The very low voltage we're using can't hurt us, but transistors, LEDs, and polarized capacitors can be damaged when the power is applied and they weren't installed correctly.

Success is when the LED flashes briefly, about 1 to 3 times per second.

Schematic

Click on the image to see a larger version.

I labelled circuit nodes (n0 to n8) to help me explain how the circuit works. (n2 was not used.)

Images

Click on the image to see a larger version.

Here are voltage waveforms of nodes n3 thru n8. (Nodes n0 and n1 waveforms are simply flat lines — not too interesting). These are taken from the circuit simulator software, and not from an oscilloscope connected to a live circuit. So they may be slightly off from reality, but still informative.

How Does It Work

Overall

This ciruit is made up of three parts, joined together:

1. The LED flasher from Project 1, Hack 01-01, slightly-modified (T1, T2, R1-R4, C1)
2. A new stage, the Voltage Booster (D1, C2)
3. The output load (LED1, R5)

Section 1 of this circuit is basically the same pulse oscillator that we used in the previous projects. We moved the output load (the LED) to a different section in the circuit. Now, because the LED was moved elsewhere, we can use a higher-value resistor for R3 (1000 ohms) — this will reduce power consumption. In the original oscillator, R3 was used to set the LED "on" current. Now, R3 doesn't need to draw as much current — R3 is needed only to setup the feedback signal (as before) and to help charge up C2 in the time between the LED-flashes.

Section 2 is our simple voltage booster stage. C2 acts as temporary storage. C2 is like a big bucket at the end of a long rope, lowered slowly down in the well (R3), scooping up some water (voltage, via D1)... and then, quickly pulled all the way up (T2) and its stored water (voltage) dumped out for us to use (into the output load, R5 + LED1). We need a large capacitor for C2 (220 μF) in order to hold a big charge that will brightly flash the LED.

Section 3 is simply the output load, R5 and LED1. R5 is used to limit the current to a level safe for the LED.

Analysis

Section 1 of this circuit is the relaxation oscillator (T1, T2, R1-R4, C1), a pulse generator, which I already analyzed in Project 1. Instead of flashing an LED, this version of the oscillator simply provides a narrow pulse of voltage to the second section. The output of section 1 is node n6.

The Time In Between

Section 2 of this circuit is a voltage booster (C2, D1). In the time between pulses, the big capacitor, C2, gets charged up. Current from the AA cell (positive terminal) flows through D1, to C2 (plus terminal), flows out of C2 (minus terminal), through R3, and back to battery (minus terminal). Because the frequency is slow enough, enough time passes to allow C2 to charge up to nearly 1.5 volts (the AA cell's voltage). The amount of time needed is determined by R3 and C2. Lesser values of R3 and/or C2 would speed up the charging rate. But if C2 is too small, the LED's flash would be too weak.

Section 3 consists of R5 and LED1, the output load. In between pulses, no current flows through these parts, so the LED is dark. The voltage at node n8 is 1.5 volts, too low to light up the LED. (The red LED requires at least 1.7 volts).

Delivering The Pulse

When section 1, the pulse generator, is in its “on” state, T2 has quickly pulled node n6 up to 1.5 volts (the AA cell voltage). Node n8, which was already at 1.5 volts, is also quickly pulled up, even higher, because C2 connects these two nodes together. So node n8 is raised to 1.5 + 1.5 = 3 volts, at least while the pulse lasts and C3 holds its charge.

Now that node n8 is at 3 volts, the LED can light up. Resistor R5 and the LED conduct current. The boost voltage stored in C3 delivers this current for a length of time set by C3 and R5. Greater values of C3 and/or R5 would extend the time the LED is lit.

Simple Mods

Flashing a Blue or White LED — Blue and white LEDs are "high-voltage" LEDs: they need a little more voltage that red, yellow, orange, or green LEDs. This circuit provides an output pulse (n8) of around 2.6 volts. Blue or white LED only begin to light up dimly at around 3 volts, and fully light up at 3.2 to 3.4 volts (this voltage depends on the exact type of blue or white LED).

So the quick answer is to double our battery from one AA cell to two AA cells in series. Two AA cells will supply 3 volts, so the circuit would output a pulse of 5.6 volts — plenty of voltage for any color LED, really. I would then increase R5 to 100 or 120 ohms, to keep the current level that's safe for the LED.

We could use a lower-voltage battery or cell, like 2 volts, but this is not a standard and easily-available battery/cell.

← Previous Article	↑	Next Article →
	Home Page