Simple Automatic Range Switching
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 circuit 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 circuit 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 circuit works: IC1 converts the voltage to be measured by the DVM module into a ground-referenced voltage. This part of the circuit 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 circuit 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 circuit 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 circuit 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 circuit 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.
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 circuit 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 circuit works: IC1 converts the voltage to be measured by the DVM module into a ground-referenced voltage. This part of the circuit 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 circuit 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 circuit 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 circuit 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 circuit 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.
Author : Rainer Reusch - Copyright: Elektor
Mini Audio Analyzer
This analyst is, a sensitive instrument, in the frequency changes and width of a acoustic signal. Thus the brightness of LED that turns on each moment of is proportional signal width, while the colour of proportionally frequency.
Mini Audio Analyzer Circuit Diagram
The circuit input sensitivity is regulated with R2, in order that in powerful signals they turn on the red LED, in the middle the yellow LED and in the low green LED. The display unit, is constituted by 3 lines of 10 LED the every, what is checked by a counter decoder (IC2). Two gates ICa-b, function as the IC2, with the R6 regulate the frequency.
When does not exist signal in the input, then no one LED does not turn on. As soon as it will be applied signal, then the LED will begin to blink depending on the rythm and the intensity of signal. Can experiment with the prices of resistances R4, R5, so that you find the value that suits in like your. Initially it can they are placed trimmer 1KO, in the place of R4, R5 and after is found the suitable value, they are replaced with permanent resistances.
What it wants it can it adds also other LED, adding other a completed same press with the IC2. The particular circuit does not have the claim, the precision of measurement input signal.
Part List
R1= 1K8Kohm
R2= 100Kohm trimmer
R3= 1Kohm
R4= 100 ohm.....1Kohm
R5= 100 ohm.....1Kohm
R6= 100Kohm trimmer
C1= 100nF 100V
D1....10= RED LED
D11....20= YELLOW LED
D21....30= GREEN LED
IC1= LM3915
IC2= 4017
IC3= 4011
Mini Audio Analyzer Circuit Diagram
When does not exist signal in the input, then no one LED does not turn on. As soon as it will be applied signal, then the LED will begin to blink depending on the rythm and the intensity of signal. Can experiment with the prices of resistances R4, R5, so that you find the value that suits in like your. Initially it can they are placed trimmer 1KO, in the place of R4, R5 and after is found the suitable value, they are replaced with permanent resistances.
What it wants it can it adds also other LED, adding other a completed same press with the IC2. The particular circuit does not have the claim, the precision of measurement input signal.
Part List
R1= 1K8Kohm
R2= 100Kohm trimmer
R3= 1Kohm
R4= 100 ohm.....1Kohm
R5= 100 ohm.....1Kohm
R6= 100Kohm trimmer
C1= 100nF 100V
D1....10= RED LED
D11....20= YELLOW LED
D21....30= GREEN LED
IC1= LM3915
IC2= 4017
IC3= 4011
Easy to Build Electronic Door Lock
Its easy to build and straightforward. This electronic door lock has a remarkable conception because it uses only one active component. How does it work?
Electronic Door Lock Circuit Diagram
To put the relay in tension the 4 buttons S1 – S4 must be pressed. If anyone of the 4 butoons S5 – S8 are pushed the relay doesn’t switch. The supply voltage must be equal to the working voltage of the relay. One transistor like BD135 can switch up to 0.5A, at the same time 2N2222 can do only 0.2A.
Enjoy this electronic door lock, its easy to build and straightforward.
Electronic Door Lock Circuit Diagram
To put the relay in tension the 4 buttons S1 – S4 must be pressed. If anyone of the 4 butoons S5 – S8 are pushed the relay doesn’t switch. The supply voltage must be equal to the working voltage of the relay. One transistor like BD135 can switch up to 0.5A, at the same time 2N2222 can do only 0.2A.
Enjoy this electronic door lock, its easy to build and straightforward.
advertisement
