Powerline Voltmeter

Here’s a rather special voltmeter that will let you measure the AC grid voltage and also see very accurately how it fluctuates around its nominal value. The voltmeter has a measuring range of around 35 V that you can centre around the nominal voltage from the grid. 

The circuit uses a bridge supplied at low voltage to make it easier to implement. The voltage available at TR1 secondary, which reflects the mains voltage multiplied by the transformer ratio (fixed and constant), is rectified by D1, filtered by C1, and stabilized at 12 V by D5. This same voltage is also rectified by D2, but this time is not stabilized and is only slightly filtered by C2 so that the circuit remains responsive. Given the value of R3, R4, and P2, the voltage at the junction of R3 and R4 can be adjusted to 12 V when the mains is at its nominal value. Any increase or decrease in it will then vary the voltage at this point and change the reading of meter M1 accordingly.

Powerline Voltmeter Circuit Diagram:

There’s no need to use a centre-zero meter, thanks to the adjustment available via P1 and P2. All you have to do is decide that when the needle is at the centre of its travel, this corresponds to 230 V. In this way, you’ll have a margin in both directions for indicating any increase or reduction. The circuit diagram gives you a choice bet ween two types of widely-available meter, but by modifying R2, and possibly R3 and R4, it can be adapted to use practically any reasonably sensitive meter.

It’s not difficult to adjust this circuit, but you do need to have access to a variable trans-former (variac). As these aren’t particularly common, contact your local technical college, for example, where you should be able to borrow one just long enough for your adjustments. Remember, most variacs are auto-transformers, i.e. they do not afford electrical isolation. To adjust set P1 to mid travel and adjust the variac to 230 V. Now adjust P2 so that the meter reads 0. Then turn the variac up to 240 V and set P1 so the meter reads full scale. Do watch out, though, as the simplicity of the circuit means there is some interaction between the two adjustments, so you’ll need to work by successive approximation to achieve the best compromise but it’ll still only take you a few minutes.

All that is left for you to do is to graduate the meter scale from, say, 215 to 240 V, and you’ll have a magnificent expanded-scale voltmeter that will let you follow the slightest variation in the AC powerline voltage.

 

Author : Christian Tavernier - Copyright : Elektor

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Simple RF Noise Source

A noise generator with a wideband output signal is always handy to have around when you're adjusting receivers and other types of HF equipment.

Simple RF Noise Source Circuit Diagram:

The noise generator circuit described here uses the base-emitter junction of a transistor (in this case a 8Fl99) operating under reverse bias. As a result, it acts as a Zener diode and generates a wideband noise signal. The noise signal passes through a 1-nF capacitor to the

output connector (female BNC), which means that its low-frequency components do not appear at the output. The 47-Q resistor gives the noise generator an  output impedance of nearly 50 Q.  You can easily fit the entire noise generator in a small metal enclosure equipped with a BNC connector. The supply voltage is not critical; anything in the range of 8 to 15 V will do.

 

Author : Fred  Brand - Copyright : Elektor

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Light Sensitive and Differential Temperature Switch

Fig.1 Precision Light- Sensitive Switch Circuit Diagram:

 

In Fig. 1 see a precision light- sensitive switch that activates when the sensed quantities go above or below pre-set values. The LDR can be any cadmium sulfide unit that has a resistance in the range 500R to 20k at the required trip level. The RV1 adjust LDR at normal light level.

Fig. 2 Differential Temperature Switch Circuit Diagram:

In Fig. 2 see a differential temperature switch circuit using ordinary silicon diodes as temperature sensing elements and responding to differentials of a fraction of degree. RV2 can be used to apply an effective offset of several degrees to the two diodes. To adjust the circuit, apply the required differential temperature to the diodes and then adjust RV2 so that the relay just turns on. The circuit responds to the relative temperatures, rather than the absolute temperatures, of the two diodes.

Part List :

R1=LDR *see text
R2-3=10Kohm
R4=2.2Kohm
R5-6-7-10=4.7Kohm
R8-9=2.7Kohm
RV1=22Kohm  pot.
RV2=1Kohm  pot.
D1-4=1N4001
D3-4=1N4148
Q1-2=BC214L
IC1-2=LM741
RL1-2=RELAY 12V >120 ohm

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Emergency Stop

The big fear of every developer of a microcontroller or computer-driven control system is that the computer or controller could crash while it is in the middle of controlling some-thing and that the output signal will remain on ‘full throttle’. In this scenario motors could continue to spin faster and faster or a heating element could become red hot, without the system taking any corrective action. In reality, any control system needs some sort of emergency stop, which will turn everything off the moment something goes wrong.

Microcontrollers or computers will usually have a spare TTL output, which can be used for this purpose. By adding a few lines of code to the program, this additional output can be made to toggle high and low periodically. This can save a lot of trouble and dam-age. Should the computer or controller crash, then the toggle signal on this output will stop as well. The circuit then, does little more than check whether this toggling (TTL-) signal is still present. The computer or controller will be turned off as soon as this control signal is missing.

Emergency Stop Circuit Diagram:

The heart of the circuit is formed by transistors T2 and T4, which follow the control signal. The accompanying capacitors C1 and C2 are charged via resistors R6 and R11. During a logic High signal, T4 will conduct and discharge ‘its’ capacitor (C2). Since T2 is pre-ceded by an inverter circuit built around T1, T2 will discharge its capacitor when the control signal is ‘low’.

Provided that the control signal changes of ten enough between high and low, both capacitors will remain nearly completely discharged and nothing else happens. If the control signal now hangs at the high level, then the capacitor connected to T2 will no longer be discharged and the capacitor voltage wil increase quickly. On the other hand, the volt-age across the capacitor connected to T4 will increase quickly if the control signal is stuck at the ‘low’ level. Via the dual diode circuit, which acts as an OR gate, T3 will be activated as soon as the voltage across one of the two capacitors builds up sufficiently. The relay that is controlled by T3, has to have a normally closed contact. The moment that the control signal stops changing, the control system will be permanently turned off via the normally-closed contact. To turn the system back on, pushbutton S1 needs to be pressed until the control signal reappears at the input of the circuit.

The circuit will operate over a wide range of power supply voltages, including 5, 9 and 12 Volts. The component values are not critical and the value of the capacitors depends on the frequency of the control signal. The time constant with a value of 10 μF amounts to 10 ms, so that the capacitors will have to be discharged at least one hundred times per second to prevent the emergency stop from operating. With higher values of capacitance the capacitors can be discharged at a proportionally slower rate. A 1N4007 can be used for the free-wheeling diode across the relay. The two diodes for the OR gate can be practically any type of signal diode. The circuit will also work with other types of transistors that have comparable specifications.

 

Author : Jacob Gestman Geradts - Copyright : Elektor

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Timer light switch

Tiny Timer Light Switch presented here is a simple transistorised electronic timer which drives a high efficiency white LED for a finite time out. This circuit is very useful for in-car reading etc. The circuit works off 12 volt dc supply. After construction, fit the unit at a suitable location inside your car and power the circuit from the in-dash standard cigar lighter socket. The timer light switch is ultra simple, economic, straight forward and self explanatory.

Timer light switch Circuit Diagram :

the timer light switch works?

Normally T1 is turned off by P1 and R2. When the trigger switch S1 is pressed the base of T1 is connected to the +12V supply via R2. Now T1 turns on and this action turns on the next transistor T2 which in turn energises the white LED (D2). Resistor R4 limits the operating current of white LED (D2).

When the switch is pressed is current also flows into capacitor C1 (through R1) and charges it. So when the switch S1 is released the charge in the capacitor C1 keeps T1 turned on until the charge has decayed away through R2 and P1. You can easily increase the ouput on time by increasing the resistance of the potentiometer P1. link

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