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Figure 1. This picture was taken before I changed C2 from
0.1 uf to 1.0 uf, and eliminated one transistor and one resistor.
The circuit shown in Figure 1 is capable of driving an LED array requiring up to several amps with a burst of flashes before coming on until power is removed. It is intended to be used as an auxiliary warning light driver. The initial burst of flashes is meant to enhance visual detection of the indicator coming on.
Figure 2. The circuit is intended to drive a set of LEDs that have internal current limiting.
The circuit in which this circuit is intended to be installed is a circuit such as a brake warning light in a car. It has no power applied until the brake light switch is closed and goes dark again when the switch opens. When power is first applied to the circuit, the LED array comes off, then blinks off several times before coming on to stay. Below is a description of the circuit.
On the left is D1, a Zener diode shunt regulator which supplies 5 volts to counter chip U1.
U1 contains and oscillator and several stages of counters, each successively dividing the its input signal by 2. The output of the first stage that leaves the chip is that of the 4th counter stage. Its frequency is 1/32 that of the oscillator. Output Q4 directly drives Q1, which when “on” (That’s when U1 output Q4 is low) turns off the output transistor, Q1.
Connected to the 5 volt supply are C2 which provides a reset pulse to the counter when power is applied. This is necessary to make the counter start at zero. D4 helps discharge C2 quickly so that the circuit can be switched on and off in quick succession.
So, when the power is first applied to the circuit, Q2 turns on, causing the LED array to come on. After the reset period which is a little less than 100 ms, the pulses into Q1 cause Q2 to shut off current to the LEDs four times before coming on to stay on. The circuit comes to a “stop” when Q7 goes high, which stops the oscillator in a state in which Q2 is left on.
The faster the oscillator, the faster the blink sequence. The photograph below shows the sequence taking approximately 750 milliseconds. It starts with that small positive step on the left hand side. The frequency is adjustable with the 250k pot, or less conveniently, by changing C3 and/or R3, R4 has very little affect on frequency and it is there to make the oscillator more temperature stable.
The number of pulses during the flashing phase can be doubled by moving the anode of D2 from U1 Q7 to U1 Q8. Or increased to four times the original number of flashes by moving the anode of D2 to U1 Q9, and so on, all the way of to Q12. To make the circuit flash continuously until power is removed, simply omit D2. (Published at Dick Cappels' project pages http://www.projects.cappels.org)
Figure 3. The photograph of a scope trace showing the drain voltage of Q2. The LED array is off until the small positive-going step at 200 milliseconds and each time the voltage goes high and after the small negative-going step at 1.375 seconds, which occurs at the instant that power is removed from the circuit.
The output transistor is rated at 8 amps –this is to give plenty of margin when driving the 250 ma LED array. The output transistor has no heatsink, and it can provide 250 ma indefinitely without any problem, but the circuit is used to supply much higher current, either the duty cycle will have to be limited or a heatsink will have to be provided to Q2.
The circuit I built operated down to 4.5 volts and over 18 volts, so it should be fine in a car, even on the coldest winter mornings, except possibly while starting the engine. Depending upon how clean the electrical system is at the point at which the circuit is connected, additional filtering of the power to the module may be required. The logic circuit's power supply is Zener regulated and that would mean that it is pretty tolerant, so try it without additional filtering. If you find you need to add filtering, do it across the input of the module rather than across the Zener diode because adding filtering across the Zener would increase the initial turn-on delay, which would be a bad thing in the case of a warning light.
This circuit is intended to illustrate the use of a CD4060/ 74HC4060 / 74HCT4060 or similar variant to flash an LED array, and should be used for the study of such circuits.
I have not evaluated this circuit to determine whether it is safe to use as an automobile brake light flasher, so I leave it to the individual to decide whether that individual is competent to judge whether the use of the circuit would cause any danger to anybody or any thing, and then to apply such a judgement before deciding whether to use this circuit in any application. In other words, as far as where this circuit can safely be used, the reader is on his own.
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