Circuit electrinic

Dynamic flip-flops ignore pulses at their inputs that are shorter than 40 ns or do not have TTL levels. This means that TTL flip-flops are poorly suited to capturing noise pulses having unknown durations and magnitudes. Anyone who has ever tried to observe very short laser pulses (15–25 ns) is familiar with this problem. By contrast, this circuit can detect impulses with widths less than 8 ns and amplitudes between +100 mV and +5 V. The heart of this circuit is formed by a MAX903, a very fast comparator with internal memory. The IC has separate supply pins for its analogue and digital portions. The analogue portion is powered by a symmetrical ±5-V supply.

This allows the detector to also handle input voltages that are negative relative to ground. The internal memory and output stage operate from a single-ended +5-V supply, so the output signal has proper TTL levels. The MAX903 (IC1) has a special internal memory circuit (latch). The latch either connects the output of the internal comparator directly to the signal output or stores the most recent TTL level and blocks the output of the internal comparator, causing the most recent TTL level appears at the output. This allows short input pulses to be stretched to any desired length. Despite its extremely short switching times, the MAX903 consumes only a modest 18 mW.

In the quiescent state, the voltage on the Latch input (pin 5) is at 1.75 V. This reference voltage is provided by LED D1, which draws its current via R2. In this state the latch is transparent, and a positive edge at the input appears will appear as a negative transition on the output after a propagation delay of 8 ns (tPD). This only happens if the peak voltage on the input is more positive than ground potential. C1 passes this change in the output voltage level to the Latch input (pin 5). As soon as the voltage on the Latch input drops below 1.4 V, the internal latch switches to the Hold state. In this state, the output is no longer connected to the comparator, and the output remains low for the duration of the latch hold time, regardless of what happens with the input signal.

The latch hold time is determined by the time constant of the C3/R1 network; it has an adjustment range of 100–500 ns. Pulses of this length can be readily observed using practically any oscilloscope. This latch function in this circuit is only triggered if the input signal has a rising edge that crosses the zero-voltage level. The internal latch remains transparent for signals in the range of –5 V to 0 V, so such pulses will not be stretched. If only positive input voltages are anticipated, the negative supply voltage is not necessary and the circuit can be powered from a single +5-V supply. A fast circuit such as this requires a carefully designed circuit board layout. All connections to the IC must be kept very short.

Decoupling capacitors C1 and C2 should preferably be placed immediately adjacent to the supply pins. Pin 3 of the IC can be bent upward and soldered directly to a length of coax or twisted-pair cable (air is still the best insulator). If a coax cable is used, the unbraided screen must not be formed into a long pigtail. It’s better to peel back a short length of the screen, wrap a length of bare wire around it and solder it directly to the ground plane. The supply traces for the analogue and digital portions must be well separated from each other, and each supply must be well decoupled, even if only a single supply voltage (+5 V) is used. The preferred solution is to use two independent voltage regulators.

This circuit uses a complementary pair comprising NPN metallic transistor T1 (BC109) and pnp germanium transistor T2 (AC188) to detect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1.

The second part of the circuit comprises popular IC UM3561 (a siren and machine-gun sound generator IC), which can produce the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC are connected to the +3V supply when the relay is in energised state, whereas pin 2 is grounded. A resistor (R2) connected across pins 7 and 8 is used to fix the frequency of the inbuilt oscillator.

Circuit Diagram

Automatic Heat Detector Circuit Diagram

 

The output is available from pin 3. Two transistors BC147 (T3) and BEL187 (T4) are connected in Darlington configuration to amplify the sound from UM3561. Resistor R4 in series with a 3V zener is used to provide the 3V supply to UM3561 when the relay is in energised state. LED1, connected in series with 68-ohm resistor R1 across resistor R4, glows when the siren is on. To test the working of the circuit, bring a burning matchstick close to transistor T1 (BC109), which causes the resistance of its emitter-collector junction to go low due to a rise in temperature and it starts conducting. Simultaneously, transistor T2 also conducts because its base is connected to the collector of transistor T1. As a result, relay RL1 energizes and switches on the siren circuit to produce loud sound of a fire-brigade siren.

Note.

• We have added a table to enable readers to obtain all possible sound effects by returning pins 1 and 2 as suggested in the table.

 Author:Sukant Kumar Behara Copyright:Circuit Ideas