THE DESIGN CRITERIA to be satisfied by this timer are that it is simple to operate, reliable, pocket sized, has an audio output and is cheap to run. As figure shows, the circuit consists of two parts: a precision digital timer and an audio oscillator. After the preset delay period, the timer circuit energises the audio oscillator. There are two operating controls. Switch S1 is first set for the required period; switch S2, the on/off switch, then initiates the delay. At the end of the period, a rapid series of pips is heard from the speaker. The time period is simply reset by switching it on again. The LED D1 is used to indicate that timi·ng is in progress and goes out when the alarm sounds at the end of the delay period. The general appearance of the prototype timer is shown in Fig. 2. Accurate timing is set by shunting VR1 and VR2 with a 2k2 resistor to obtain a time delay of 40 seconds. Firstly, the timer circuit based on IC1. This integrated circuit is a precision timer device, (Ferranti ZN1034E), in a 14 lead DIL package.
The frequency of an on chip oscillator is determined by an externally—connected capacitor and resistor. Pulses from this oscillator are fed through a 12 stage binary divider which switches the output stage after 4095 counts. During the count-out period the drain current is a low 5mA or so, and the oscillator frequency is independent of supply voltage in the range 5V to 450 V (an . on-chip voltage regulator is used). Capacitor C1 has a fixed value of 4.7;¢`F and the resistors R1 and R2 are selected empirically to provide time intervals of 1 hour and 2 hours, respectively. Values of approximately 270 k12 and 540 kS2 are required. Of course, great precision in the time intervals required is not necessary for the application in mind for the timer, but the great advantage of using this timer chip instead of the ubiquitous ‘555 IC is that large- value resistors and capacitors are not required for delays of an hour or so. For connections shown in the figure, the time delay in seconds is given by · T = 2736C1R1 At the end of the delay period, output pin 2 goes positive and output pin 3 goes negative. Thus, during the timing period, the positive voltage on pin 3 drives on the LED and the negative voltage on pin 2 keeps off transistor Trl . When the timer counts out the positive voltage rise at pin 2 switches on Tr1 which provides current for the oscillator based on the integrated circuit lC2.
lC2 is the well known dual timer device, the 556 consisting of two 555 timers in the same chip. Each timer is wired as an astable multibrator which are cross-coupled by resistor. R10. The low frequency oscillator based on R6, R7 and C3 modulates the high frequency audio oscillator based on R8, R9 and C4 to give a rapid series of pips from the speaker LS. The values of these frequency determining resistors and capacitors, and the value of R10, can be experimented with to obtain the audio signal required. For instance, if the value of R10 is raised from 2.2 H2 to 6.8 ki) the audio note changes from a succes- y sion of pips to a two—tone alarm. lf it is intended to use this device ‘ as a parking timer, ·it might be best to set the period just short of 1/2 hours, say by 10 minutes or so, to give your- self time to get back and redeem the situation, before the dreaded piece of paper descends on your windscreen.
The frequency of an on chip oscillator is determined by an externally—connected capacitor and resistor. Pulses from this oscillator are fed through a 12 stage binary divider which switches the output stage after 4095 counts. During the count-out period the drain current is a low 5mA or so, and the oscillator frequency is independent of supply voltage in the range 5V to 450 V (an . on-chip voltage regulator is used). Capacitor C1 has a fixed value of 4.7;¢`F and the resistors R1 and R2 are selected empirically to provide time intervals of 1 hour and 2 hours, respectively. Values of approximately 270 k12 and 540 kS2 are required. Of course, great precision in the time intervals required is not necessary for the application in mind for the timer, but the great advantage of using this timer chip instead of the ubiquitous ‘555 IC is that large- value resistors and capacitors are not required for delays of an hour or so. For connections shown in the figure, the time delay in seconds is given by · T = 2736C1R1 At the end of the delay period, output pin 2 goes positive and output pin 3 goes negative. Thus, during the timing period, the positive voltage on pin 3 drives on the LED and the negative voltage on pin 2 keeps off transistor Trl . When the timer counts out the positive voltage rise at pin 2 switches on Tr1 which provides current for the oscillator based on the integrated circuit lC2.
lC2 is the well known dual timer device, the 556 consisting of two 555 timers in the same chip. Each timer is wired as an astable multibrator which are cross-coupled by resistor. R10. The low frequency oscillator based on R6, R7 and C3 modulates the high frequency audio oscillator based on R8, R9 and C4 to give a rapid series of pips from the speaker LS. The values of these frequency determining resistors and capacitors, and the value of R10, can be experimented with to obtain the audio signal required. For instance, if the value of R10 is raised from 2.2 H2 to 6.8 ki) the audio note changes from a succes- y sion of pips to a two—tone alarm. lf it is intended to use this device ‘ as a parking timer, ·it might be best to set the period just short of 1/2 hours, say by 10 minutes or so, to give your- self time to get back and redeem the situation, before the dreaded piece of paper descends on your windscreen.
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