SMALL CIRCUITSCOLLECTION

82 Elektor Electronics 12/2000

H. Baggen (text)design © 2000 VellemanThe 555 is a timer IC that has so many applications that it isnot possible to name all of them. Here, a 555 forms the heart ofa simple signal generator, which can produce square wave,rounded square wave, triangular and sinusoidal signals. A wirebridge or jumper can be used to connect one of these signalsto an output buffer. The circuit produces a fixed output fre-quency of 1 kHz, and the output level can be adjusted between0 and approximately 200 mVeff using a potentiometer.The 555 is wired as a squarewave generator, with the outputfrequency determined by the values of R1, R7 and C3. Thesymmetry of the squarewave depends on the ratio of R1 andR7. The output signal goes via the voltage divider R8 and R2and jumper J1 to transistor T2, which is configured as an emit-ter follower. A low-impedance signal is available at the emit-ter of T2. This is finally brought to the output via the couplingcapacitor C8 and potentiometer RV1.The squarewave is integrated by the RC time constant of R4and C5, and the resulting rounded squarewave is available atJ2, from which it can be buffered by T2. A second RC stage(R5 and C6) makes the signal transitions even more gradual,so that something resemblinga triangular waveform isobtained. This signal is avail-able at J3. Finally, the tops ofthe triangular waveform arerounded off by the amplifierstage around T1, due to thepresence of C2. This signal isavailable at J4.The current consumption ofthe circuit is approximately10 mA, so it can easily bepowered by a 9-V battery.Diode D1 provides reverse-polarity protection, so that the555 and the transistors willnot go up in smoke if you acci-dentally connect the batterythe wrong way around.

(005099-1)

The order code of this Velleman kit is MK105.

For further information, visit http://www.velleman.be

E1

9V

D1

1N4007

C9

1µ16V

R1

1k

R7

15

k

R8

4k

7

R21k

R9

10

0k

R10

10

0k

R3

1k

R4

10k

R5

10k

R6

10k

R11

1M

C1

10n

C3

47n

C4

47n

C5

47n

C6

47n

C7100n

C2

10n

C8

1µ16VT1

BC547B

T2

BC547B

47k

RV1

IC1

TR

DIS

THR

OUT555

CV

2

7

6

4

R

3

5

8

1

J4

J3

J2

J1

U

0V5

0V5

1V6

0V66

005599 - 11

TIMER

max p p

555 Signal Generator Kit031

By G. KleineThe Over-the-Top type of operational amplifier is ideal for useas a current sense for battery charger applications. The designdescribed here can be used with chargers for rechargeablebatteries (Lead/acid or NiCd etc). The 5 V operating supply forthe circuit is derived from the battery on charge. The circuituses a sense resistor R8 to determine the value of current flow-ing in or out of the battery. An LED output shows whether thebattery is charging or discharging and an analogue output dis-

plays the battery charge or discharge current. The circuit canalso be altered to shown different ranges of charging current tocater for higher capacity cells.

IC1a and IC1b together with T1 and T2 form two currentsources, which produce a voltage across R5. The voltageacross R5 is proportional to the current through resistors R8and R1 (for IC1a) or R8-R3 (for IC1b). The current sourceformed by IC1a and T1 is active when the batteries are dis-charging and IC1b and T2 is active when the batteries are

Battery Charger Display032