Universal Symmetric Power Supply

 

This power supply has been specially designed for the 20 th -order filter described elsewhere in this issue, but it can also be used for a legion of other opamp circuits. The supply voltage is set to ± 17.5 V, in light of the maximum output level of the filter. This benefits the signal to noise ratio. The specified absolute maximum supply voltage for most opamps is ± 18 V, and we have intentionally kept a bit below this limit. The transformer is one of a series made by Hahn (model UI 30), so the circuit can be easily adapted for higher power levels by using a different transformer. All transformers in this series have the same footprint (53 X 44 mm), with only the height changing according to the power capacity. The series consists of 3, 4, 6, 10 and 16-VA models, which are respectively 16.3, 18.3, 21.8, 27.7 and 37.6 mm high. There are two secondary windings, with standard voltages of 2 X 6, 2 X 9, 2 X 12, 2 X 15 and 2 X 18 V. We chose a 4 VA transformer with 2x18V secondaries for this application. Certain models are also available from other manufactures, but the locations of the secondary connections are different. The circuit board layout can accommodate two different types.

The circuit is based on the well-known LM317 and LM337 voltage regulators. Since the output voltages are set by voltage dividers, any voltage between 1.25 V and 40 V is possible. In case you don't already know, the formula for the positive output voltage (LM317) is

V out = 1.25*(1+R2/R1) + I adj *R2

The same formula applies to the negative regulator, using R3

and R4 instead. Capacitors C5 and C6 increase the ripple suppression to 80 dB. Depending on the application and the output power, it may be necessary to use heat sinks for the regulator ICs. The power supply has a simple mains filter to suppress common-mode interference. This is primarily needed if the supply is used to power sensitive circuits. The coil is a Siemens type that has been used in many other Elektor Electronics projects. Dl acts as a mains voltage indicator. The indicated value of the fuse, both in the diagram and on the circuit board, is 32 mA (slow). This value will have to be modified for higher power levels (as will the label on the circuit board!). With lower output voltages and larger output currents, the filter capacitors C9 and CIO must be made larger. The working voltage can then be reduced, so the physical dimensions will probably remain the same. 

The PCB shown here is available ready-made through the Publishers' Readers Services.

Analog Opto Coupler

 

It is sometimes necessary to make an electrically isolated connection in a circuit. An optocoupler is usually the key component in such a situation. In most optocouplers, a single lightemitting diode (transmitter) and a single photodiode (receiver) are optically coupled inside the package. This solution is satisfactory for transferring digital levels (such as the control signals for a thyristor), since only two logical states (LED on or LED off) have to be transferred. An exact (analogue) coupling is thus not necessary.

If an analogue voltage must be transferred, then it is important that the voltages at the input and the output closely track each other. To make this possible, the transmitter and receiver must employ comparable components that are incorporated into an analogue circuit. The type CNR200 and CNR201 opto-couplers that are available from Agilent (formerly HewlettPackard) contain all the essential components for such a function. There are two photodiodes and one LED in a single package, with an optical coupling between the LED and one of the photodiodes. The schematic diagram shows how the transmitter LED is optically coupled to the photodiode in the receiver. The remaining photodiode is incorporated into the transmitter and ensures that the characteristic of the transmitter amplifier is the same as that of the receiver. Assuming a supply voltage of 5 V, analogue voltages in the range of 0 to 3 V can be readily transferred. The isolation voltage between the input and output of this optocoupler is 1000 V. The value that can be achieved in practice depends on the printed circuit board layout

Single-Opamp 10-MHz Bandpass Filter

 

A bandpass filter is usually used to pass frequencies within a certain frequency range. If a high-performance opamp is used, such a filter can also be used at relatively high frequencies. As shown in the schematic diagram, here we have chosen an OPA603, which is a fast current-feedback opamp with a 100 MHz bandwidth for gain values between 1 and 10 (0 to 20 dB). If the circuit only has to handle a narrow range of frequencies, as in this case, the gain can be increased.

 With a current-feedback opamp, just as with an ordinary opamp, the negative feedback between the output and the inverting input determines the gain. In addition, the impedance of the feedback network determines the open-loop gain and the frequency response. With the component values shown in the schematic diagram, signals outside the passband are attenuated by 22 dB. The centre frequency of the filter is 10 MHz. As indicated by the printed formula, the centre frequency can easily be altered. However, keep in mind that 10 MHz is roughly the maximum frequency at which this circuit can be used. The circuit can be powered by a supply voltage of ± 15 V.

Infra-Red Light Barrier

 

This is a short-range light barrier for use as an intruder alarm in doorposts, etc. The 555 in the transmitter (Figure 1) oscillates at about 4.5 kHz, supplying pulses with a duty cycle of about 13% to keep power consumption within reason. Just about any infra-red LED (also called IRED) may be used. Suggested, commonly available types are the LD271 and SFH485. The exact pulse frequency is adjusted with preset PI. The LEDs are pulsed at a peak current of about 100 mA, determined by the 47 Q series resistor.

In the receiver (Figure 2), the maximum sensitivity of photodiode D2 should occur at the wavelength of the IREDs used in the transmitter. You should be okay if you use an SFH205F, BPW34 or BP104. Note that the photodiode is connected reverse-biased! So, if you measure about 0.45 V across this device, it is almost certainly fitted the wrong way around. The received pulses are first amplified by Tl and T2. Next comes a PLL (phase lock loop) built with the reverenced NE567 (or LM567). The PLL chip pulls its output, pin 8, Low when it is locked onto the 4.5 kHz 'tone' received from the transmitter. When the (normally invisible) light beam is interrupted (for example, by someone walking into the room), the received signal disappears and IC1 will pull its output pin High. This enables oscillator IC2 in the receiver, and an audible alarm is produced.

The two-transistor amplifier in the receiver is purposely overdriven to some extent to ensure that the duty cycle of the output pulses is roughly 50%. If the 2 transmitter is too far away from the receiver, overdriving will no longer be guaranteed, hence IC1 will not be enabled by an alarm condition. If you want to get the most out of the circuit in respect of distance covered, start by modifying the value of R2 until the amplifier output signal again has a duty cycle of about 50%. The circuit is simple to adjust. Switch on the receiver, the buzzer should sound. Then switch on the transmitter. Point the transmitter LEDs to the receiver input. Use a relatively small distance, say, 30 cm. Adjust PI on the transmitter until the buzzer is silenced

Switch the receiver off and on again a few times to make sure it locks onto the transmitter carrier under all circumstances. If necessary, re-adjust PI, slowly increasing the distance between the transmitter and the receiver

Battery Discharger

 

The battery discharger published in the June 1998 issue of this magazine may be improved by adding a Schottky diode (D 3 ). This ensures that a NiCd cell is discharged not to 0.6-0.7 V, but to just under 1 V as recommended by the manufacturers. An additional effect is then that light-emitting diode D 2 flashes when the battery connected to the terminals is flat.

The circuit in the diagram is based on an astable multivibrator operating at a frequency of about 25 kHz. When transistor T 2 conducts, a current flows through inductor L lf whereupon energy is stored in the resulting electromagnetic field. When T 2 is cut off, the field collapses, whereupon a counteremf is produced at a level that exceeds the forward voltage (about 1.6 V) of D 2 . A current then flows through the diode so that this lights. Diode D 1 prevents the current flowing through R 4 and C 2 . This process is halted only when the battery voltage no longer provides a sufficient base potential for the transistors. In the original circuit, this happened at about 0.65 V. The addition of the forward bias of D 3 (about 0.3 V), the final discharge voltage of the battery is raised to 0.9-1.0 V. Additional resistors R 5 and R 6 ensure that sufficient current flows through D 3 . When the battery is discharged to the recommended level, it must be removed from the discharger since, in contrast to the original circuit, a small current continues to flow through D 3 , R 2 _ R 3, and R5.R6 until the battery is totally discharged

The flashing of D 2 when the battery is nearing recommended discharge is caused by the increasing internal resistance of the battery lowering the terminal voltage to below the threshold level. If no current flows, the internal resistance is of no consequence since the terminal voltage rises to the threshold voltage by taking some energy from the battery. When the discharge is complete to the recommended level, the LED goes out. It should therefore be noted that the battery is discharged sufficiently when the LED begins to flash.