Optical Pulse Generator

This little aid was originally designed to test the Shutter Time Meter. This meter was specifically designed for ‘analogue’ SLR cameras. In order to measure the exposure time of a camera accurately, it will first have to be checked with a well-defined signal first. This circuit was designed for that purpose. But the circuit can also be used if you need a well-defined pulse for some other purpose. The circuit is build around a trio of standard logic ICs. Firstly a 74HC4060 (IC1) is used to provide a quartz crystal accurate reference for the duration of the pulses. For the crystal frequency we choose the common 4.096 MHz value.

Picture of the project:

To test all the ranges of the shutter time meter, we choose three different pulse lengths in three different decades, namely: 1 / 2 / 4 / 10 / 20 / 40 / 100 / 200 / 400 ms. With jumper J1 you select a frequency of 1000, 500 or 250 Hz (see table). The frequency is then passed on to J2 and the dual decade counter IC2 (a 4518). This does not need to be a fast HC-type, since the frequency is at most 1 kHz. With J2 the frequency can be reduced by 1, 10 or 100 times. This frequency is then applied to IC3 (a 5-stage Johnson-counter). This has been set up in such a way that in the end there appears only one single pulse at the output.

Circuit diagram:

Optical Pulse Generator Circuit Diagram

The advantage of the Johnson-counter is that each output is free from glitches and has a duration that is exactly equal to the period of the clock input. We choose Q2 as the output. Q4 is used to stop the counter. Q0 is only active if we push the reset-button S1. IC3 will then start to count. To ensure that the reset does not affect the duration of the pulse, a differentiating RC-network R4/C3 generates a short reset pulse. R3 ensures that C4 is discharged after releasing S1. Also, just to be sure, we don’t use the second counter output but use the third one instead. For the same reason, to stop the counter we use the fifth output.

Parts and PCB layout:

Especially with longer times you will notice that the pulse will arrive at the output a short time after pressing the switch. R5 drives a current of nearly 20 mA through D1. D1 provides sufficient light for this application to trigger the receiver diode in the shutter time meter. An unusually fast type was selected for the LED, which, with a switching time of 40 ns, has practically no influence on the length of the pulse. If you would like to use another LED then you will have to look closely at the switching time.

This needs to be small compared to the duration of the pulse. If you want to use the circuit with a logic level output then you can just omit D1. If necessary, the pulse lengths can be changed be selecting another crystal frequency. The current consumption in the idle state is less than 2 mA. In our prototype, while the circuit is delivering a pulse, the current consumption increases briefly to about 18 mA. Do not forget the wire link under IC2 when assembling the circuit.

Resistors
R1 = 1k
R2,R3 = 1M
R4 = 10k
R5 = 180
Capacitors
C1,C2 = 33pF
C3 = 10nF
C4,C5,C6 = 100nF ceramic, lead pitch 5mm
Semiconductors
D1 = HSDL-4230
IC1 = 74HC4060
IC2 = 4518
IC3 = 74HC4017
Miscellaneous
S1 = pushbutton, make contact, 6mm
J1,J2 = 3-way pinheader with jumper
X1 = 4.096MHz quartz crystal 1 wire link.

Source: Elektor Electronics

Read more

Long-Interval Pulse Generator

A rectangular-wave pulse generator with an extremely long period can be built using only two components: a National Semiconductor LM3710 supervisor IC and a 100-nF capacitor to eliminate noise spikes. This circuit utilises the watchdog and reset timers in the LM3710. The watchdog timer is reset when an edge appears on the WDI input (pin 4). If WDI is continuously held at ground level, there are not any edges and the watchdog times out. After an interval TB, it triggers a reset pulse with a duration TA and is reloaded with its initial value. The cycle then starts all over again. As a result, pulses with a period of TA + TB are present at the RESET output (pin 10).

Circuit diagram:

Long-Interval Pulse Generator Circuit Diagram

As can be seen from the table, periods ranging up to around 30 seconds can be achieved in this manner. The two intervals TA and TB are determined by internal timers in the IC, which is available in various versions with four different ranges for each timer. To obtain the desired period, you must order the appropriate version of the LM3710. The type designation is decoded in the accompanying table. The reset threshold voltage is irrelevant for this particular application of the LM3710. The versions shown in bold face were available at the time of printing. Current information can be found on the manufacturer’s home page (www.national.com). The numbers in brackets indicate the minimum and maximum values of intervals TA and TB for which the LM3710 is tested. The circuit operates with a supply voltage in the range of 3–5 V.

Author: Gregor Kleine
Copyright: Elektor Electronics

Read more

CMOS Crystal Frequency Multiplier

Crystals usually operate at fundamental frequencies up to about 15 MHz. Whenever higher frequencies are required a frequency multiplier is placed after the crystal oscillator. The resulting output signal is then a whole multiple of the crystal frequency. Other frequency multipliers often use transistors, which produce harmonics due to their non-linearity. These are subsequently filtered from the signal. One way of doing this is to put a parallel L-C filter in the collector arm. This filter could then be tuned to three times the input frequency. A disadvantage is that such a circuit would quickly become quite substantial.

This circuit contains only a single IC and a handful of passive components, and has a complete oscillator and two frequency triplers. The output is therefore a signal with a frequency that is 9 times as much as that of the crystal. Two gates from IC1, which contains six high-speed CMOS inverters, are used as an oscillator in combination with X1. This works at the fundamental frequency of the crystal and has a square wave at its output. A square wave can be considered as the sum of a fundamental sine wave plus an infinite number of odd multiples of that wave. The second stage has been tuned to the first odd multiple (3 x).

We know that some of our readers will have noticed that the filter used here is a band-rejection (series LC) type. Worse still, when you calculate the rejection frequency you’ll find that it is equal to the fundamental crystal frequency! The fundamental frequency is therefore attenuated, which is good. But how is the third harmonic boosted? That is done by the smaller capacitor of 33 pF in combination with the inductor. Together they form the required band-pass filter. (The same applies to the 12 pF capacitor in the next stage.) Through the careful selection of components, this filter is therefore capable of rejecting the fundamental and boosting the third harmonic! Clever, isn’t it?.

The output in this example is a signal of 30 MHz. The inverter following this stage heavily amplifies this signal and turns it into a square wave. The same trick is used again to create the final output signal of 3 times 30 MHz = 90 MHz. At 5 V this circuit delivers about 20 milliwatt into 50 R. This corresponds to +13 dBm and is in theory enough to drive a diode-ring balanced mixer directly. The circuit can be used for any output frequency up to about 100 MHz by varying the component values. When, for example, an 8 MHz crystal is used to obtain an output frequency of 72 MHz (9 x 8 = 72), the frequency determining inductors and capacitors have to be adjusted by a factor of 10/8.

You should round the values to the nearest value from the E12 series. Another application is for use in an FM transmitter; if you connect a varicap in series with the crystal, you can make an FM modulator. An added bonus here is that the relatively small modulation level is also increased by a factor of 9. Crystals with frequencies near 10 MHz are relatively easy to find and inexpensive, so you should always be able to find a suitable frequency within the FM band. A crystal of 10.245 MHz for instance would give you a frequency of 92.205 MHz and 10.700 MHz results in an output of 96.300 MHz. You may find that the circuit operates on the border of the HC specifications. If this causes any problems you should increase the supply voltage a little to 6V.

Author: Gert Baars
Copyright: Elektor Electronics

Read more

Pulse Frequency Modulator

The pulse width of the compact pulse cum frequency modulator can be varied by altering the change-over point of comparator IC1 with a control voltage via resistor R1. The hysteresis of the IC is determined by resistors R3 and R4. The control voltage also causes the frequency of the present circuit to be altered. When the input voltage is 0 V, the frequency is a maximum: in the present design this is about 3.8 kHz. The level of the output voltage is ±12–13 V. The more the change-over point has been shifted with the control voltage, the longer it will take before the potential across capacitor C1 has reached the level at which IC1 is enabled.

Pulse Frequency Modulator Circuit Diagram

When the control voltage is larger than the zener voltage, the oscillator ceases to work. The maximum period is 25 ms, which may be adapted by altering the value of C1. This will, of course, also alter the maximum frequency. The duty cycle is inversely proportional to the control voltage. The minimum pulse width attainable at the lowest frequency is about 6 µs. The modulator draws a current not exceeding 5 mA.

Read more