# Discrete-Circuits (Page 3)

## Simple Water operated relay

This was done in my early days, i have upgraded it, it ought to work, reduce the number of transistors to make it less sensitive, also a lower value in place of 10M will reduce its sensitivity, use clamping diodes to protect.

More such ideas at – Hobby Hound – Hobby, DIY, Do it Yourself Projects.

BD139 is used to drive the relay as it has good Ic. So you can even use a low ohm relay. If a Relay resistance is high its quality is higher, its power consumption is less and it needs thinner wire SWG-AWG. T2 and T3 form a darlington pair which drives T1. LED1 shows that the water level has reached the top of tank and also that the Relay is energised. D1 a freewheeling diode. R3 10M ensures that the high gain input does not float, yet the low leakage current thru the water is not drawn away by the 10M. R2 limits base current in case water is saline.

The Source file in EAGLE format is here del00006.zip

## Ohmmeter – Simple Resistance Measurement

Measurement of resistor values in circuit configurations are required to be made often, as these might have changed in value due to various tolerance ranges, and hence could be the cause of faults. Likewise the resistance of components used in a circuit, may need to be known. In such cases the measurement of resistance is a must.

The circuit used for measurement of voltage can be modified to measure the value of the unknown resistance. The principle followed is the measurement of voltage drop across the resistance when a constant current flows through it. In the voltage measuring circuit, the unknown resistance is connected to the same input terminals and the switch SR is operated. Then a constant d.c. current from the collector of transistor T I is passed through resistor R16 to the unknown resistance which is grounded. The voltage drop across the unknown resistor is proportional to the value of the resistance as current is maintained constant. This d.c. voltage drop is measured after proper calibration.

For the constant current source a high gain, low leakage, pnp silicon transistor (T1) is required. The range selector switch Rs, which connects the positive voltage to the constant current source enables measurement of resistances in 5 decades i.e. 200 ohms, 2 kilo-ohms, 20 kilo-ohms, 200 kilo-ohms and 2 mega-ohms.

According to the range of resistance being measured the switch Rs also selects the decimal point of the displays in the DPM. A resistor R limits the current to the decimal point of the LED displays. Transistor T I is biased by resistor R17 and variable present VR5. As this preset sets the value of current in transistor T1, it has to be adjusted for calibrating the resistance range. Once the calibration is over, the resistance value is directly read on the DPM.

(This is scanned-ocr from my earlier file, some mistakes corrected – delabs)

## Flashing Mains Neon Lamp

Here is a Neon Flasher circuit (untested) for a user request at Circuits FAQ. This can be built into a switchboard or a gadget for indicating Live Power.

D1-C1 form a simple half-wave rectifier, The Cap charges to peak voltage and can store charge for a long time if there is no bleeder. So while building it take extra care. This forms a DC supply across C1. C1 is a Plastic High-voltage cap, IN4007 has a 1KV rating, so it is ok for 230V rectifier.

R1 Charges C2 and when C2 reaches 60-80V depending on Neon, the neon breaksdown. C2 Discharges, Neon Recovers, The C2 starts charging again and so on and on. It Oscillates, probably in a Ramp Waveform. But do not use your Scope on this, you will regret it a lot. This is a live circuit and needs a special probe.

“Oh, i will put the probe it in 10M mode” will not do. The ground clip of the probe goes to Electrical Earth which is ‘connected’ to Neutral in the mains wiring. So you put the earth crocodile clip on the live point. There will be flashes and fireworks. So you need to isolate both terminals of scope. Please use your costly equipment with great care.

For the 1 Meg use two 470K in Series for 230V AC, that is safer. The circuit is live, so take precautions. The 0.47 Micro Farad can be increased if you want a slow flash. If the Mains 50/60 Hz Flicker is too much, the 1 uF can be made 2 uF, or use 4 – 1N4007 as a bridge rectifier.

 From Schematics of delabs

User Feedback –

R1 of 4.7M and C2 of 0.47uF Works well at 230V AC. Try your own Combination. Less than 1M may damage Neon.

## Voltage Doubler

This is a Simple Voltage Doubler to boost 3V battery voltage to power some low-power 5V circuits. In needs a Clock input with high fan-out. You can use the 74HCT540 in parallel. Work with the Audio frequency range and see.

If you feed a Low Impedance Square wave, like output of 555, which has adequate Source-Sink Punch. to Input A, then it is quite possible you get double the voltage at B on No load.

## FET Current Source and MOSFET

This is a constant current source using a FET. This is the most simple replacement to series resistor to limit current. The N-Channel FET BF256C can give 15mA current.

Simple Methods

Before you get to use chips, experiment with some methods, which will help you learn about the LEDs better. The first is just One Resistor in series. This is to Limit the max current in a Series LED Chain. If you have a Regulated Supply with a Fixed Voltage, then you can use this method.

Let us take a 12V SMPS, Each HB White LED has a drop of around 3.2 (please see datasheet). If you put 3 LEDs in series it is a drop of 3 X 3.2 = 9.6 V.

12V – 9.6V = 2.4 V. This is the drop across the Resistor, let us keep the current at 20mA for a Long life for LED. Some LEDs will get damaged at 30mA some take more that that. We now have LED Modules which can take even 1 or 2 A.

V/I = R as per OHM. 2.4V/20mA = 120 E or Ohms.

How Hot? W = VI Power in Watts. 20mA x 2.4V = 48mW. This is where you lose the Money. Keep it low, else the Green Goblin will frown. Unless you want LED Lighting to double up as a Room Heater, Nice idea if you are in the Artic.

Now you have a chain of 3 LED with one R, make many such chains and put it in parallel to around 70% of SMPS capacity. If you have 20 Strips of 3 LEDs each, 20 X 20mA = 400mA. You will need a 12V 600mA SMPS .

MOSFET Drive for LED Constant Current

Let us assume, you have a supply that is varying and not stable. Then use a SMPS and Resistor as shown above. Closely matching the LED Chain to the SMPS voltage to keep the heat loss minimum. At Low voltages the above idea may not work. So you can try a MOSFET circuit shown.

You can use Transistors too but The Heat is more, as the Drop is more. When you use batteries, you cannot afford to lose even 0.5V. So the MOSFET is the answer.

This circuit is a nice design idea about LED drive with low voltage and watts burden. In combination with a Joule Thief – and PWM you can make many White LED utilities like Lanterns and Flashlights. PWM is to modulate brightness and also Extend LED Life.

The Essence is The LED has to have a long life, constant current is the answer. The Efficiency has to be High, Switching MOSFET is the answer. For just a LED or Two, you do not need to bother about Efficiency, but Constant Current, No Compromise. Why? Because it is in the Absolute Maximum Rating.

“You do not cross the road, when the light is RED. You do not Drive Faster, than the Speed Limit. You do not Eat, more than you can Digest.You do not Stress the Absolute Max in the Ratings.”

The Elektrik Jedi