9V Automatic Battery NiCd Charger

This automatic NiCd charger for 9V NiCd batteries is using 555 timer properties and is very easy to build. Why is an automatic 9 volts NiCd battery charger? Because you can leave the battery for charging as much as you like: it will be always completely charged and ready for use when is needed. It wont be overcharged and it will not discharge.

9V Automatic Battery NiCd Charger Circuit Diagram :

With the values presented in the circuit diagram, the battery charger NiCd circuit is suitable for 6V and 9V batteries. 9 volt types with 6 and 7 cells are charging with 20mA; P1 must be adjusted so that the NiCd charger disconnects after 14 hours. Window inferior level is set at 1V below this value with P2. 5V battery type with 4 or 5 cells are charged at 55mA. Again, with P1 adjust the NiCd charger circuit so it disconnects after 14 hours. Window inferior level must be set at 0.8V below this value.

 

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Automatic Night Light Feeds Directly From the AC Line

There are many approaches to the problem of activating a light when it becomes dark, and a recent Design Idea covers this topic (Reference 1). Some approaches require a dc power supply and an electromechanical relay, but a better approach involves feeding the device directly from the ac line, minimizing the number of components 

(Figure 1).

Figure 1. The photoresistor activates the TRIAC and the load when darkness falls.

The heart of the device is a light-sensitive cadmium-sulphide resistor, PR, with a resistance of approximately 200 kΩ in the dark and decreasing to a few kilohms in the light. PR and capacitor C1 form an ac-voltage divider. In daylight, the voltage across PR is too low to generate the required gate-trigger current to turn on bidirectional ac switch Q1, thus keeping the load – usually a lamp – off. When it becomes dark, PR’s resistance rises, resulting in an increase in the TRIAC’s gate current that triggers the TRIAC and lights the lamp.

The schema uses inexpensive, off-the-shelf components, including the VT90N1 photoresistor; a 0.1-μF, 275V capacitor; and an L2004F61 TRIAC with a load current of 4A rms, a peak blocking voltage of 200V, and a gate-trigger current of 5 mA. The exact specifications of these components are not critical; you could use others instead.

Editor’s note:
Attributes worth mentioning include the fact that the capacitor introduces a phase shift, which places the peak of the gate voltage close to the zero crossing of the load’s sine wave for optimum turn-on timing. Another benefit is thermal hysteresis, which occurs due to the reduction of the required triggering voltage and current as the TRIAC warms up after the initial turn-on. 

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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 schema 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 diagram). 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 schema 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 schema 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

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Automatic Range Switching Wiring diagram Schematic

You can pick up a 3½-digit digital volt-meter module nowadays for a little as a couple quid. This is a simple and expensive way to fit out a piece of equipment with an instrument. Most modules are based on the well-known ICL7106 IC . They operate from an ordinary 9-V battery, and they only provide a fixed measuring range (200 mV or 2 V). The accessory schema described here converts a DVM module into a voltmeter with 20-V and 200-V measuring ranges, with the added bonus of automatic range switching.

This requires a ground-referenced symmetrical supply voltage (±5 V) instead of a battery. An inexpensive TL431C is also used to generate an adjustable reference volt-age from the supply voltage. The schema described here uses an LCD module with a fixed measuring range of 200 mV. It has three pins for driving the decimal point; two of them are used here.

Automatic Range Switching Schematic

This is how the schema works: IC1 converts the voltage to be measured by the DVM module into a ground-referenced voltage. This part of the schema is based on a design idea from Carsten Weber [1] that was pub-lished in the June 2005 issue of Elektor Electronics.

If the input voltage is less than 20 V, the voltage divider formed by R1 and R4 reduces it by a factor of 100. Transistor T2 is cut off, so R3 has no effect on the division ratio. The voltage at the junction of voltage divider R8/R13 is 200 mV because the open-collector output of comparator IC2A is in the high-impedance state. If the input voltage rises above 20 V, IC2A changes state and the voltage at the junction of voltage divider R8/R13 drops to less than 20 mV. In response to this, the out-put of comparator IC2B goes high and T2 conducts. R3 is now connected in parallel with R4.

This yields a division factor of 1000 (200-V range). Of course, the larger division factor also causes the input voltage of IC2A to drop. To prevent this comparator from changing back to its previous state (which would cause the schema to act like a sort of oscillator), the value of R10 must be chosen such that the voltage at the junction of voltage divider R8/R13 is less than 20 mV, as previously mentioned. The calculated value (with R10 in parallel with R13) is approximately 9.6 mV. In practice, the value is around 18 mV due to the resistance of the output transistor of the comparator.

This means that the schema will switch back to the lower voltage range when the input voltage drops below approximately 18 V. The amount of hysteresis can be set by adjusting the value of R10. However, the schema will oscillate if the value is too high. Film capacitors C1, C3 and C4 sup-press noise and create a certain amount of inertia for range switching. This prevents frequent back-and-forth switching in the threshold region.

The other two comparators of IC2 sup-ply mutually complementary output levels that depend on the measuring range. The associated decimal points of the DVM module are driven via p-channel FETs.The schema has two trimpots: P1 is used to correct for the offset voltage of the operational amplifier (IC1), while P2 is used to set the threshold level for range switching For this purpose, first adjust the trimpot to produce the maximum possible reference voltage (around 3.4 V). Next apply an input voltage that causes a display reading of 19.99 (which ideally means 19.99 V). Now turn P2 until the measuring range switches.

As a check, reduce the input voltage to force the measuring range to switch back, and then slowly increase the input voltage again. The ideal setting is reached when the measuring range switches before the DVM module displays an ‘overrange’ indication. Link

Author : Rainer Reusch - Copyright: Elektor

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