The input signal of the UAF 2115 derived from a reed con-
tact or a proximity switch (dropout oscillator) placed, for
example, at the drive shaft of the gearbox and which sup-
plies one rectangular pulse per shaft rotation.

The monostable flip-flop is triggered by the trailing edge of
the input signal. Possible bouncing of the reed contact
therefore does not produce faulty indications, since such
bouncing coincides with the unstable period of the flip-flop.
The shape of the input pulses is irrelevant, provided that
the predetermined upper and lower thresholds are reliable
attained.

The pulse duration produced by the monostable is deter-
mined by the RC network Rg_1q, Cg_13. It can be varied over
wide limits and is capable if beeing adapted to the input
frequency.

The rectangular pulses control the current source (meter
driver), the frequency divider and the pulse amplifier. The
‘speed-dependent output current of the meter driver is indi-
cated by a moving-coil meter giving the car's speed. The
mean output current from Pin 5 can be adjusted externally
by means of a resistance from Pin 4 to ground so as to
equalize tolerances of the meter.

The pulse output, Pin 3, is an n-p-n transistor with open
collector to control a taximeter. In the event of a short
circuit or overload the internal current limiter comes into
action and, after a short time, disconnects the output.

The frequency divider, whose division ratio can be selected
by means of Pin 11 to be 25, 25 or 27, supplies pulses for
controlling the stepping motor buffer.

Pin 11 connected to pin 10: Division factor of divider is 27,
Pin 11 connected to ground: Division factor of divider is 25,
Pin 11 open: Division factor of divider is 25.

The stepping motor buffer generates two interlaced pulse
trains with a puise duty factor of 37,5 % which drive the two
stepping motor widings connected to Pin 12 and Pin 13.
The duty cycle of the motor currents is shown in Fig. 4.
Some stepping motor coils may act as an inverting trans-
former. The switching-off coil may induce a negative vol-
tage in the other coil. Be careful that the voltage at pins 2,3
and 12 do not become more negative than ~0.5 V.

Each of the two outputs and the pulse output, Pin 3, are
provided with Integrated flyback diodes, the cathodes of
Which beeing connected to Pin 1. The p-n-p current source
(output Pin 7) can be used to feed the sensor which pro-
duces the speed dependent input pulses.

In order to protect the integrated circuit against high vol-
tage peaks from the car supply system, an external filtering
network for the supply voltage must be provided. The mo-
tor supply current does not flow through the resistor of the
filter network. Reliable starting of the stepping motor is
thus ensured even in the case of low battery voltage.