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Pace alo em to sie, em joe PI dot onte ore [O Ls low. “hermistos Soo Ts enutes that lhe |, tempesacre ot tã rot tscuatos on la o Adao a atu Teto Dere lhe relay il sc eos jul plaugu 1996 — 718) pat or la art verso Sé E) Te eita dest oe CMOS logio sito poimo MA fumos opor Dutpat The mena siga! bica ana 720 — elektor july/august 1976 18 simple pulse generator . sonshora A pulse generator is an ex- 1 tremely useful tool for investi- gating the dynamic behaviour of logic circuits. The circuit described here is based on the versatile 555 timer, and provides variable pulse length and repetition frequency. zener diodes DI and D2 protect the in- put of Ná against excessive input volt- ages to the “ext trig' and “gate” inputs. With S4 in the 'single-shot” position the set-reset flip-flop comprising N1/N2 can be set by pressing the single-shot button. The output of N3 will thus go low. triggering IC2 once. Construction AM components with the exception of the switches, potentiometers and timing capacitors Cl to Cl1 and Cl4 to C24 r E E” «DR ext trig rep. rate ue su f BEMpAbadi oo single shot. LI ie o, & ICI is connected as an astable multi- vibrator, whose repetition frequency depends on R1, R2, PI and the switched capacitors Cl to C11. The switched capacitors provide coarse control of the pulse interval, while PJ provides fine control. The output of IC] is taken via a differentiating network R4/C12 to the mid-position of S4. With S4 in the mid-position the pulse train from IC1 is fed through N4 and NS to the trigger input of IC2, which is connected as a monostable multi- vibrator. This is repetitively triggered by the input pulses and the output pulse length is controlled by capaci- tors C14 to C24 and by P2. The output of IC2 is buffered by the inverters N6 to N9 and a TTL compatible output and its complement are provided at the outputs of N7/N9 respectively. For testing other logic families such as HLL or CMOS a variable amplitude output is provided by the emitter follower T1. The amplitude may be adjusted by P3. If this option is adopted a 15 V collector supply to T1 is re- quired. Tn addition to a continuous pulse train the circuit also has facilities for external trigger, external gating of the pulse train and single-shot operation. With S4 in the mid-position the pulse train from ICI may be interrupted by applying a logic “0” to pin 5 of N4. With Sá in the “ext' position the output of IC1 is disconnected and IC2 may be triggered from an external source. The Ni... Ná =1C2=7400 im N5-. N10 = IC4 = 7406 722 — elektor july/august 1976 21 dark room aid «+ som 22 battery indicator 23 wide band frequency doubler aid in the dark room to ensure correct exposure time without effort. Before the paper is placed under the enlarger, the amount of light is measured by means of an LDR. This is the light-sensitive element. The LDR makes up one branch of a bridge circuit, which is formed by the LDR, RI, R2 and part of P2. The other branch consists of PI, R3, R4 and a part of P2. With P2 in centre position, and P1 adjusted to a resistance value lower than that of the LDR, the voltage at the + input of the op-amp is lower than the voltage at the — input. With this con- dition, the output voltage of the op-amp is negative, and DJ] lights up. On the other hand, when the resistance of P1 is higher than that of the LDR, the output is positive, and D2 lights. If the voltages on both inputs are equal, both LEDs light at half brilliance. This is due to the fact that hum is picked-up by the high sensitivity op-amp. Thus the circuit indicates when the resistance values of the LDR and PJ are equal. When the LDR is placed under the enlarger, its resistance value will correspond to the light intensity. PI is now adjusted until the bridge is balanced, after which the exposure time can be read from a calibrated scale attached to Pl. By adjusting P2 slight changes to the bridge balance are poss- ible. In this way it is possible to intro- duce corrections for different sensi- tivities of the photographic paper. This dark room aid has only one draw- back: the scale calibration of Pl can only be obtained by spending some evenings in the dark room. But, of course, for the real enthusiast, this is no problem at all! The LDR must be mounted in a flat holder and be partly covered by a mask. This method allows spot measurement 2 This circuit is intended as an and also helps adapt the LDR to the circuit. To calibrate the unit, first of all ensure that the bridge can be balanced with PI over the entire range, from extreme light to extreme dark. If not larger or smaller apertures in the LDR mask can be tried. After this, by using an ohm-meter, Pl can be provided with a scale. For example: P1 = 500 S2 gives one second, 1 kS2 gives 2 seconds, and so on until P1 k9 which corresponds to 64 seconds. By means of test strips and an “average grey” negative, the exposure time can now be brought into accord- ance with the sensitivity of the photo- graphic paper by means of P2. To this end, P2 is also provided with a scale with the type numbers or gradations for different papers. The control range of P2 is equal to 4 stops. If this scale is shifted too far towards one of the extreme positions, the LDR mask must be changed. Since only a small area of the picture is measured by the LDR, it is possible to determine the contrast, and choose the type of paper accordingly. 2 One drawback of primary cells and batteries is that they often go flat at the most inconvenient times. The circuit described here can do nothing about dead batteries, but it can give timely warning that the batteries must be replaced or charged soon. The battery indicator is suitable for voltages between 3 and 15 V; the threshold value can be adjusted with P1 When the voltage drops below the set value, the LED (D1) lights up, giving an indication that the batteries need attention. (O 12v RES ê ah R1 R3 KT 41 The circuit uses only a few components, requiring very little space so that it can be built into miniature radio sets. As long as the battery is O.K., the cir- cuit draws about 1 to 3 mA; when the LED lights up, this increases to 5 to 15 mA. To save current, S1 can be added which can be either a toggle switch or a push-button switch. Frequency doubling devices 2 usually employ a transistor stage operating in class C. In its output circuit there is a tuned circuit which resonates on the second harmonic. The amount of fundamental frequency rejectionis = governed by the Q of this tuned circuit. However, when a frequency doubler based on the SO 42-P is used, a 100 ohm pot is used to reject the fundamental. At 10 MHz, and as long as the input signal does not exceed 30 mV rms level, 40 dB fundamental suppression can be obtained. The output capacitance of the circuit is 6p. 9... 15V o] eis O AE PES eb k or so42P ME E [To $ E elektor july/august 1976 — 723 24 liquid level indicator 25 aerial booster 26 variable slope filter 2 This circuit was originally intended as a water level indi- cator for use by blind persons, to give an audible indication when a cup, bowl or other container was full. It will function with any liquid that will conduct electricity, such as beer, tap water, tea, milk. It will, of course not function with distilled or de-ionised water. The circuit has other applications such as a rain sensor (when used with a suitable probe) The circuit is extremely simple. The in- put of NI is normally held low by a 1M resistor. When the probes are im- mersed in a conducting liquid the input of N1 goes high, so the output goes low and the output of N2 goes high, en- abling the astable multivibrator N3/N4, which switches T1 and T2.on and off to produce a tone from the speaker. An open collector transistor output is also provided to drive a relay or other cir- cuit, Probe construction for level sensing and for rain sensing are shown in figure 2. The level sensor probes should preferably be made of stainless steel wire for ease of cleaning, and the circuit housing should be watertight in case of acgidents. q N1--N4=4011 2 A very convenient way of improving the performance of a not-so-sensitive FM tuner is the addition of a VHF pre-amplifier stage in front.of the existing equipment. Satisfactory gain improvement can be obtained if the booster stage is designed using a “Tg6ss BF200 (BE180) ps E 12v transistor having good VHF properties, such as the BF 200 or BF 180. These transistors also have good noise figures. This booster design was tried in front of a variety of receivers with good results. The average voltage gain figure proved to be about 12 dB. The preamplifier design is quite straight- forward, using a common base configur- ation conditioned for minimum noise. 75 ohm antenna systems can be coupled straight to CI. For 300 ohm systems a matching transformer LI can be made, by winding a 2 turn primary and a 1 turn secondary on a ferrite bead. Use 0.3 mm enamelled wire (SWG33). The output matching transformer is wound on a 6 mm (4”) diameter coil form. It should have a ferrite slug with a permeability gr = 12. Using 0.9 mm enamelled wire (SWG21) the primary (L2A) has 4 turns while the secondary (L2B) has 2 turns. The circuit is not particularly critical to operate, provided that due care is taken in the construction of the amplifier and long connections are avoided. The transistor screening pin can be connected to the base, as shown in the drawing, or straight to the p.c. board common. The only circuit requiring adjustment is the primary of the output trans- former, L2/C4. This adjustment is carried out by tuning the FM set to a station at approximately 95 MHz. C4 is then set to minimum capacitance, after which L2 is set for maximum signal strength by adjusting the slug. Tuning is completed by the adjustment of C4, also for maximum signal strength. 2 Most RC noise filters show a fixed roll-off (slope) at frequencies above the cross-over fre- quency. Such filters with a single RC network usually have a 6 dB per octave roll-off. Admittedly, with such filters, noise and high frequency distortion is made less obtrusive. However, not only the noise is effected, an important part of the high frequency content of the original signal may also have been wiped out. In such cases a filter with adjustable roll-off slope would be an asset. Figure 2 shows a noise filter circuit with a cross-over frequency of approximately 7 kHz and a variable slope adjustable between O and approximately 25 dB/ mL 1 DO elektor july/august 1976 — 725 29 simple headphone amplifier od sy TI functions as the voltage detector; via R2 and D2 the emitter of this transistor is held at a potential about equal to the TTL-supply voltage. If the base voltage now becomes about 0.7 V higher than the emitter voltage, T1 begins to conduct and triggers Th via T2 and R4. When the thyristor is triggered, relay Re is energized and cuts off the TTL-supply voltage. In addition a LED is connected parallel to the relay to give the user visual warning that the voltage is too high. Diede D1 limits the negative base/ emitter voltage of T1 to a value of about 0.3 V. Furthermore, D4 and R6 in combination with the thyristor, create a heavy load for the TTL-voltage. Since a “broken-down' supply is no longer stabilized, the TTL-voltage immediately drops to a safe value under influence of the additional load when the thyristor is triggered Consequently, the response time of the relay does not play a significant part in this protective circuit. Digde D4 prevents interaction of the two supply voltages. The supply for the protective circuit (8 .: . 16 V) can be obtained from the non-stabilized out- no) put (electrolytic smoothing capacitor) of the 5 V TTL-supply. The thyristor is reset by switching off the supply. Diode D4, shown as a BY 103, can be replaced by any type that can handle at least 2 A. Thyristor Th must be a 3 Amp. type, and the operating voltage of relay Re should be about 10 V. 2 This headphone amplifier is intended for use with high impedance headphones (greater than 300 S2) such as the Sennheiser HD414 and HD424. It is extremely simple and uses only two transistors per channel. The circuit comprises a voltage amplifier Ti, driving an emitter follower T2, which provides the necessary low out- put impedance to drive the “phones”. The circuit was intended to be driven by a control amplifier that can provide a nominal | V r.m.s. output, such as the TCA730/740 circuit published in Elektor 8. It was found that a gain of between 3 and 4 in the headphone amplifier would provide an adequate volume level in the headphones. The gain is determined by the ratio R3:R4 and is about 3.7. With such a small gain the distortion is fairly low due to the large amount of negative feedback at Resistors: R1=330k R2=56k R3=6k8 R4=1k8 R5=470 92 Capacitors: Ci=1u C2=2204/16V C3=224/16V Semiconductors: T1 = BC1O9C T2=BC108B 2 of each component for stereo, except C3. the emitter of T1. The unbypassed emitter resistor R4 ensures that the impedance at the base of T1 is high (in fact [1 + hFE Ti ] times R4) so the actual input impedance of the circuit is largely determined by the biasing resistors R1 and R2, and is greater than 40 k. The amplifier can thus be driven without difficulty by the control ampli- fier. The output impedance of 6k8 at the collector of T1 is stepped down by the emitter follower T2, so that the head- phones can be driven. The frequency response of the circuit is determined mainly by the input and output coupling capacitors Cl and C2 and is substantially flat between 20 Hz and 20 kHz. The nominal supply voltage of the circuit is 15 V but any supply volt- age between 12 and 18 V may be used. The printed circuit board layout is ar- ranged so that the input connections line up with the outputs on the TCA730/740 control board. 9º 02 oo Oasis gu É 726 — elektor july/august 1976 30 current source « rrasccin 31 tapped code lock » cow 32 wien bridge oscillator 3 This circuit features a Darlington pair T2/T3 joined by TI to complete a negative feedback loop. This can be explained as follows. Let it be assumed that, for one reason or another, the source current I has a tendency to increase. This would in- crease the potential across R3, causing the TI base-to-emitter potential to rise and thereby the current through T1 to a TI-T3=TUN 965 * see text increase. As a consequence the potential across R1 would also rise, thereby re- ducing the T2/T3 base-to-emitter potential, which in turn would strongly counteract the tendency of the source current to increase. R2 has the effect of rendering the source current relatively independent of small power supply voltage fluctuations. This current is practically exclusively determined by R3. The R3 resistance can be found from the equation R3 = 600/1 with R3 expressed in ohms and'I in milliamps. It is much easier to open a door 1 with a code lock than always having to carry a heavy bunch of keys about. Besides, keys can get lost, and code locks can't. The diagram in figure 2 shows a code lock that can be opened with a simple three-digit code. However, this lock operates in a slightly unusual way. With this lock, three figures must be touched simultaneously (i.e. with three fingers). One finger in the wrong place, and the relay cannot be energized. As figure 1 clearly shows, it is imposs- ible to deduce from the lock's construction how it should be operated. This feature, together with the enormous number of possibilities, makes it impossible for an outsider to open the lock. The code for the example is 9-11-4, When this code is tapped, the inputs of N2, N3 and N4 become IEEE Õ y E: logic “0”; so, the three inputs of N8 and the output of N5 become logic “1”. If none of the other taps are touched, all inputs of N7 will be logic “1”, which causes the relay to be energized and the LED to light up. The relay can be used to switch an electric door opener. LU) * see text NI—N6=7404 N7-N8=7410 32 This oscillator uses a 741 opamp (or 1/4 ofa LM 324)asits = active component. If features an interesting and effective method to stabilise the output amplitude. A pair of reverse-parallel strapped diodes is used instead of the more conventional NTC resistor, incandescent lamp or FET. The circuit will produce 10 V peak-to-peak signal with no more than 0.2% distortion The amplitude stabilising network consists of the diode pair DI/D2 together with Pl, R3, R4 and R7.P1 sets the output amplitude. P1 is connec- ted into the circuit in such a way that when its resistance is turned too low it will kill the oscillations. The ampli- tude range of PI may be shifted to some extent by increasing the resistance of R7. The oscillation frequency can be found 1 27RC inwhichR=RI=R2=10k,and C=Cl=C2. by using the equation f = 
