iAccess--an intelligent access control system （3）

Published:2011/7/28 0:52:00 Author:Li xiao na From:SeekIC

Design by I. Joostens

Schematic diagram

Buffers, logic and drivers

A ULN2803 (IC4) is used as a driver IC for the various indicator LEDs and the output relay. This IC contains eight open-collector drivers, each of which can switch a maximum current of around 500 mA.

IC11 and IC12 are included because the microcontroller I/O pins cannot supply enough current in the high state to properly drive a ULN2803, and because we prefer to use active-low signals at the microcontroller level (since all I/O pins are high during a reset). These ICs have a dual role. First, they act as buffers between the microcontroller and IC4. and second, they form a logic circuit that also provides the necessary signal inversion. This allows the microcontroller software to be simplified. 50-Hz signals are present on pins 6 and 8 of IC11 for driving a door opener.

Door opener interface

The door opener is driven directly by IC5, an L6202. This IC, which is actually intended to be used as a motor driver, contains a full H-bridge using MOSFET technology. It can handle a continuous current of 1.5 A and peak currents up to 5 A at a maximum voltage of 48 V. Both halves of the H bridge are driven by the 50-Hz signals from the microcontroller. Whether the door opener actually receives any current depends on the signal at the
Enable input of the IC. This signal is set high by the microcontroller after a valid key has been presented. However, this does not happen just like that; there is also protective circuitry in the signal path.

The first protective circuit is built around IC6, which is a 555 wired as a monostable timer with a period of approximately 30 seconds. When the signal on pin 2 of ICll goes high, Tl is switched on via R25. This causes the trigger input of IC6 (pin 2) to be briefly pulled to ground until C24 has been charged. The rising edge on pin 2 of ICll does not affect the Reset input of IC6, which is active low, so the timeout starts running at this point. If the signal on pin 2 of ICll returns to the low level during the 30-second interval, the falling edge on pin 4 of IC6 will reset IC6.

The direct effect of this is that the output signal on pin 3 exactly follows the Enable signal as long as it does not stay high longer than 30 seconds; otherwise IC6 will independently disable the output signal. This prevents the door opener from being enabled for an extended length of time, which would be detrimental to both the coil of the door opener and the lead-acid battery.

A second protective circuit prevents IC5 from being overloaded and acts as a sort of current limiter. The current flowing though the door opener and IC5 reaches ground via sense resistor R24. The voltage across R24 is fed via R31/C34 (for noise suppression) to NAND gate IC13a, which is wired as an inverter. As soon as the voltage on pins 1 and 2 of IC13 reaches half the supply voltage level, IC13 will see this as a ’high’ logic level and pin 3 will go low. This occurs with a current of approximately 1.14 A (2.5 V -^ 2.2 Q). This will cause timer IC7 to be triggered via pin 2 and start a timeout of around 1 second. Pin 3 of IC7 will go high, and overload indicator D21 will be illuminated. The Enable signal on pin 2 of IC5 will be pulled low via IC13b-d, interrupting the current. After 1 second, IC7 will reset itself, and if the overload condition has been corrected the circuit will again be operational. If the overload is still present, IC7 will immediately be retriggered after resetting itself. The net result is a series of short current pulses at a one-second rate, which will not harm IC5.

Power supply

The power supply consists of a charging circuit built around voltage regulator IC9 and a small lead-acid storage battery rated at 12 V/1.2 A. To keep the ripple component of battery charging voltage as small as possible, smoothing capacitor C18 has been generously dimensioned. D16 prevents current from flowing in the reverse direction during a mains outage. Diodes D11-D15 are included to provide temperature compensation. If the temperature increases, the voltage across the diodes will decrease, which will cause the charge voltage on the battery to increase. The charge voltage can be adjusted within certain limits using P2.

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