Flashing-LED Battery-status Indicator

Parts:

R1,R7__________220R  1/4W Resistors
R2_____________120K  1/4W Resistor
R3_______________5K6 1/4W Resistor
R4_______________5K  1/2W Trimmer Cermet or Carbon
R5______________33K  1/4W Resistor
R6_____________680K  1/4W Resistor
R8_____________100K  1/4W Resistor
R9_____________180R  1/4W Resistor

C1,C2____________4µ7  25V Electrolytic Capacitors

D1____________BAT46  100V 150mA Schottky-barrier Diode
D2______________LED  Red 5mm.

Q1____________BC547   45V 100mA NPN Transistor
Q2____________BC557   45V 100mA PNP Transistor

B1_______________5V to 12V Battery supply

---

Comments:

A Battery-status Indicator circuit can be useful, mainly to monitor portable Test-gear instruments and similar devices.
LED D1 flashes to attire the user's attention, signaling that the circuit is running, so it will not be left on by mistake. The circuit generates about two LED flashes per second, but the mean current drawing will be about 200µA.
Transistors Q1 and Q2 are wired as an uncommon complementary astable multivibrator: both are off 99% of the time, saturating only when the LED illuminates, thus contributing to keep very low current consumption.
The circuit will work with battery supply voltages in the 5 - 12V range and the LED flashing can be stopped at the desired battery voltage (comprised in the 4.8 - 9V value) by adjusting Trimmer R4. This range can be modified by changing R3 and/or R4 value slightly.
When the battery voltage approaches the exhausting value, the LED flashing frequency will fall suddenly to alert the user. Obviously, when the battery voltage has fallen below this value, the LED will remain permanently off.
To keep stable the exhausting voltage value, diode D1 was added to compensate Q1 Base-Emitter junction changes in temperature. The use of a Schottky-barrier device (e.g. BAT46, 1N5819 and the like) for D1 is mandatory: the circuit will not work if a common silicon diode like the 1N4148 is used in its place.

Note:

• Mean current drawing of the circuit can be reduced further on by raising R1, R7 and R9 values.

Push-bike Light

Parts:

R1_____________Photo resistor (any type)
R2______________22K  1/2W Trimmer Cermet or Carbon type
R3_______________1K  1/4W Resistor
R4_______________2K7 1/4W Resistor
R5_____________330R  1/4W Resistor (See Notes)
R6_______________1R5   1W Resistor (See Notes)

D1____________1N4148  75V 150mA Diode

Q1_____________BC547  45V 200mA NPN Transistor
Q2_____________BD438  45V 4A PNP Transistor

LP1____________Filament Lamp(s) (See Notes)

SW1_____________SPST  Toggle or Slider Switch

B1______________6V or 3V Battery (See Notes)

---

Comments:

This design was primarily intended to allow automatic switch-on of push-bike lights when it gets dark. Obviously, it can be used for any other purpose involving one or more lamps to be switched on and off depending of light intensity.
Power can be supplied by any type of battery suitable to be fitted in your bike and having a voltage in the 3 to 6 Volts range.
The Photo resistor R1 should be fitted into the box containing the complete circuit, but a hole should be made in a convenient side of the box to allow the light hitting the sensor.
Trim R2 until the desired switching threshold is reached. The setup will require some experimenting, but it should not be difficult.

Notes:

• In this circuit, the maximum current and voltage delivered to the lamp(s) are limited mainly by R6 (that can't be omitted if a clean and reliable switching is expected). Therefore, the Ohm's Law must be used to calculate the best voltage and current values of the bulbs.
• For example: at 6V supply, one or more 6V bulbs having a total current drawing of 500mA can be used, but for a total current drawing of 1A, 4.5V bulbs must be chosen, as the voltage drop across R6 will become 1.5V. In this case, R6 should be a 2W type.
• At 3V supply, R6 value can be lowered to 1 or 0.5 Ohm and the operating voltage of the bulbs should be chosen accordingly, by applying the Ohm's Law.
  Example: Supply voltage = 3V, R6 = 1R, total current drawing 600mA. Choose 2.2V bulbs as the voltage drop caused by R6 will be 0.6V.
• At 3V supply, R5 value must be changed to 100R.
• Stand-by current is less than 500µA, provided R2 value after trimming is set at about 5K or higher: therefore, the power switch SW1 can be omitted. If R2 value is set below 5K the stand-by current will increase substantially.

Two-wire Lamp Flasher

Parts:

R1______________6K8  1/4W Resistor
R2____________270K   1/4W Resistor
R3_____________22K   1/4W Resistor

C1____________220µF   25V Electrolytic Capacitor
C2_____________10µF   25V Electrolytic Capacitor

D1___________1N4002  100V 1A Diode

Q1____________BC557   45V 100mA PNP Transistor
Q2____________BD139   80V 1.5A NPN Transistor

LP1___________Existing filament Lamp: any type in the range 3-24V 10W max.

SW1___________Existing On-Off switch

B1____________Existing V DC source: any type in the range 3-24V
                                    suited to the lamp adopted

---

Device purpose:

This circuit was designed to provide that continuous light lamps already wired into a circuit, become flashing. Simply insert the circuit between existing lamp and negative supply.
Especially suited for car or panel pilot lights, this device can drive lamps up to 10W.

Notes:

• Break lamp(s) to negative supply connection(s), then insert the circuit between existing lamp(s) connection(s) and negative supply (respecting polarities!).
• C1 value can be varied from 100 to 1000µF or higher, in order to change flashing frequency.
• Although rather oversized, this circuit can also drive any LED, providing a suitable resistor is fitted in series with the light emitting device.
• The resistor should lie in the 47R to 2K2 range, depending on supply voltage.