Automatic TV Lighting Switch

Automatic TV Lighting Switch. The author is the happy owner of a television set with built-in Ambilight lighting in the living room. Unfortunately, the television set in  the bedroom lacks this feature. To make up for this, the author attached a small lamp to the wall to provide background lighting, This makes  watching television a good deal more enjoyable, but it ’s  not the ideal solution. Although the TV set can be  switched off with the remote  control, you still have to get out of bed to switch off the lamp.

Circuit diagram :

Automatic TV Lighting Switch Circuit Diagram

Consequently, the author devised this automatic lighting switch that switches the background light on and off along with the T V set. The entire circuit is fitted in series with the mains cable of the TV set, so there’s no need to tinker with the set. It works as follows: R1 senses  the current drawn by the TV  set. It has a maximum value  of 50 mA in standby mode,  rising  to around   500 m A  when  the  set  is  operating. The voltage across R1 is limited by D5 during negative  half- cycles  and  by  D1– D4  during positive half-cycles.  T he  voltage  across  these  four diodes charges capacitor C1 via D6 during positive  half-cycles. This voltage drives the internal LED of solid-state switch TRI1 via R2, which causes the internal triac to conduct and pass the mains voltage to the lamp.   Diode D7 is not absolutely necessary, but  it is recommended because the LED in the  solid-state switch is not especially robust  and cannot handle reverse polarisation. Fuse  F1 protects the solid-state switch against  overloads. T he  value  of  use d  here  (10 Ω)  for  resistor R1 works nicely with an 82-cm (32 inch)  LCD screen.

With smaller sets having lower  power consumption, the value of R1 can be  increased to 22 or 33 Ω, in which case you  should use a 3-watt type. Avoid using an  excessively high resistance, as otherwise TRI1 will switch on when the TV set is in standby mode.  Some TV sets have a half-wave rectifier in the  power supply, which places an unbalanced  load on the AC power outlet. If the set only  draws current on negative half-cycles, the cir-cuit won’t work properly. In countries with  reversible AC power plugs you can correct  the problem by simply reversing the plug. Compared with normal triacs, optically cou-pled solid-state relays have poor resistance  to high switch-on currents (inrush currents).

For this reason, you should be careful with  older-model TV sets with picture tubes (due  to demagnetisation circuits). If the relay fails,  it usually fails shorted, with the result that the TV background light remains on all the time. If you build this circuit on a piece of perf-board, you must remove all the copper next  to conductors and components carrying  mains voltage. Use PCB terminal blocks with a spacing of 7.5 mm. This way the separation between the connections on the solder  side will also be 3 mm. If you fit the entire  arrangement as a Class II device, all parts of  the circuit at mains potential must have a  separation of at least 6 mm from any metal  enclosure or electrically conductive exterior  parts that can be touched.

Author :Piet Germing - Copyright : Elektor

Automatic Bicycle Light

T his  automatic  bicycle  light  makes cycling in the dark much  easier (although you still need  to pedal of course). The circuit  takes  the  ambient  light  level  into account and only turns on  the light when it becomes dark.  The light is turned off when no  cycling has taken place for over  a minute or if it becomes light  again. The biggest advantage of  this circuit is that it has no manual controls. This way you can  never ‘forget’ to turn the light  on or off. This makes it ideal for  children and those of a forgetful  disposition.

Bicycle Light Image :

To detect when the bicycle is  used (in other words, when the  wheels turn), the circuit uses a  reed switch (S1), mounted on  the frame close to the wheel.  A small magnet is fixed to the  spokes (similar to that used with  most  bicycle  speedometers),  which  closes  the  reed  switch  once for every revolution of the  wheel. Whilst the wheel turns,  pulses are fed to the base of T1  via C1. This charges a small electrolytic capacitor (C2). When it is  dark enough and the LDR there-fore has a high resistance, T2  starts conducting and the lamp  is turned on. With every revolution of the wheel C2 is charged  up again. The charge in C2 ensures that T2  keeps conducting for about a minute after  the wheel stops turning. Almost any type of  light can be connected to the output of the  circuit.

Circuit diagram :

Automatic Bicycle Light Circuit Diagram

Part List :

Resistors
R1 = 1MΩ (SMD 0805)
R2,R4 = 100kΩ (SMD 0805)
R3,R6 = 1kΩ (SMD 0805)
R5 = LDR e.g. FW150 Conrad Electronics # 183547
Capacitors
C1 = 1µF 16V (SMD 0805)
C2 = 10µF 16V (SMD chip type)
C3 = 100nF (SMD 0805)
Semiconductors
T1 = BC807 (SMD SOT23)
T2 = STS6NF20V (SMD SO8)
Miscellaneous
S1 = reed switch (not on board) +
2-way right angle pinheader
BT1 = 3–12V (see text)

With a supply voltage of 3V the quiescent  current when the reed switch is open is just  0.14 μA. When the magnet happens to be in  a position such that S1 is closed,  the current is 3 μA. In either case  there is no problem using batteries to supply the circuit. The  supply voltage can be anywhere  from 3 to 12 V, depending on the  type of lamp that is connected. Since it is likely that the circuit  will be mounted inside a bicycle light it is important to keep  an eye on its dimensions. The  board has therefore been kept  very compact and use has been made of SMD components. Most  of them come in an 0805 pack-age.  C2 comes in a so called  chip version. The board is single sided with the top also acting as the solder side.

The print outline for the LDR (R5)  isn’t exactly the same as that of  the  outline  of  the  LDR  mentioned  in  the  component  list.  The outline is more a general one  because there is quite a variety  of different LDR packages on the  market. It is therefore possible  to use another type of LDR, if for  example the light threshold isn’t  quite right. The LDR may also be  mounted on the other side of the  board, but that depends on how  the board is mounted inside the  light. For the MOSFET there are also many alternatives available, such as the FDS6064N3 made  by   Fairchild ,  the  SI4864 DY  made by  Vishay Siliconix , the IR F74 0 4 made by IR F or the NTMS 4N01R 2G  made by ONSEMI. The reed switch also  comes in many different shapes and sizes; some of them are even waterproof and come with the wires already attached.

For the supply connection and  the connection to the lamp you  can either use PCB pins or solder the wires directly onto the  board. The soldered ends of the  pins can be shortened slightly so that they  don’t stick out from the bottom of the board.  This reduces the chance of shorts with any metal parts of the light. Do take care when you use a dynamo  to  power the circuit the alternating voltage must first be rectified! The same applies to  hub dynamos, which often also output an  alternating voltage.