Meter drive inhibit circuit

A meter drive inhibit circuit is provided whereby an erroneous jump of the meter pointer upon system power-up or subsequent microcomputer reset operation is inhibited. A pulse generator is utilized to produce output pulses relating to the rate of rotation of a drive shaft in a typical tachometer application. Such output pulses are provided to a meter drive circuit which in turn provides an output to an electromechanical meter. Such electromechanical meter will display an output related to the time average value of pulses input to the meter drive circuit. An inhibit circuit stage is connected to the meter drive. A microcomputer is utilized to sense the power-up condition of the meter circuit and activate the inhibit stage. When activated, the inhibit stage will prohibit the meter drive from providing an output to the meter and thereby prevent the erroneous jump of the meter pointer upon system power-up.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a meter drive circuit and, more 
particularly, to an inhibit circuit for use in a meter drive circuit. 
In an electronic meter such as a tachometer or speedometer, a pulse 
generator is utilized to provide an output pulse at a frequency related to 
the rate of rotation of an appropriate shaft in the vehicle in which the 
meter is mounted. Such a pulse generator usually comprises a general 
purpose integrated circuit which receives a triggering pulse from a sensor 
circuit associated with the rotating shaft. The output from the pulse 
generator is usually a series of squarewave pulses which are supplied to a 
meter drive circuit. This meter drive circuit can take many forms, but 
typically comprises a semiconductor device which in turn is connected to 
the actual meter. The meter usually comprises an electromechanical meter 
having a needle movement whose deflection is related to the time average 
value of the pulse input from the meter drive circuit. 
A problem with such electronic meters is that the pulse generator may 
provide an unwanted output pulse upon turning on or power-up of the 
device. Such an unwanted pulse would cause the meter needle to jump to an 
erroneous high value and then fall back to a zero value. This is an 
undesirable condition, and accordingly, it is an object of the present 
invention to provide a meter drive inhibit circuit to prevent such 
erroneous meter pointer movement. 
SUMMARY OF THE INVENTION 
The present invention provides a meter drive inhibit circuit wherein an 
erroneous meter movement which would occur upon turning on the current 
through the meter is eliminated. 
In electronic meters such as tachometers and speedometers, a pulse 
generator is utilized to provide a series of pulses at a frequency related 
to the rate of rotation of the vehicle drive shaft. Such output pulses are 
provided to a meter drive circuit to provide an input to an 
electromechanical meter movement. Such electromechanical meter is usually 
of a deflecting needle type, the deflection of which depends on the time 
average value of the input signal to the meter drive circuit. Typically, 
the pulse generator receives a triggering input from a sensor circuit 
associated with the drive shaft of the vehicle. 
Usually the meter drive circuit comprises a semiconductor device such as a 
transistor which, when turned on or in a conducting state, will allow the 
pulse generator output to pass to the meter. The meter drive inhibit 
circuit of the present invention utilizes circuitry to turn off this meter 
drive transistor to inhibit any pulse from the pulse generator from 
producing any meter movement. 
To insure that the inhibit circuitry is operating at the power-up stage, 
and then turns off to permit meter operation after any chance of an 
erroneous pulse has ended, a computer usually comprising a microprocessor 
is utilized. An output from this microprocessor is tied to the base of a 
semiconductor device such as a transistor, usually called the inhibit 
transistor. At a power-up stage of operation of the meter circuit, the 
microprocessor output goes to a high impedence state during this reset 
condition. This causes the inhibit transistor to be turned on to a 
conducting state which thereby provides a path for the pulse generator 
ouptput away from the meter driving transistor. The microprocessor is 
internally programmed to remain in such logic one high impedance condition 
for a preselected time long enough to insure that no incorrect pulse is 
transmitted to the meter. The microprocessor output will, after such 
delay, be set to a low state to turn the inhibit transistor off. This will 
permit the output from the pulse generator to turn on the meter drive 
transistor which in turn permits normal deflection and operation of the 
meter. 
In particular, the present invention provides a meter drive inhibit circuit 
comprising a pulse generator having an output, a meter drive section 
connected to said pulse generator output and itself having an output, a 
meter connected to said meter drive output, a computer having an output 
and an inhibit stage connected to said computer output and having an 
output connected to said meter drive section, said computer capable of 
providing an output signal to said inhibit stage whereby said inhibit 
stage inhibits said meter drive section from receiving said pulse 
generator output.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a block diagram of the meter drive inhibit circuit 
of the present invention is shown. Pulse generator 10 provides an output 
connected to meter drive circuit 12. Meter drive circuit provides an 
output to meter 14, usually comprising an electromechanical deflecting 
needle type meter, although electrical analogs of such meters would be 
operable in this circuit. Computer 16 usually comprises a microprocessor 
and is connected to inhibit stage 18. Upon receiving the appropriate 
output from computer 16, inhibit stage 18 will provide an output to meter 
drive 12 thereby prohibiting any output from meter drive 12 to meter 14. 
Typically, such inhibit is provided under a turning on or power-up 
condition of the circuit. After a preselected delay, computer 16 will 
change the output to inhibit stage 18 and accordingly an appropriate 
output will be provided to permit meter drive 12 to operate meter 14 under 
normal conditions. 
Referring now to FIG. 2, a more detailed drawing of the present invention 
is shown. Pulse generator 20 is connected through resistor 22 and lead 32 
to the base of drive transistor 24. The collector of drive transistor 24 
is connected through lead 26 to meter 28. Another terminal of meter 28 is 
connected to a positive voltage source 30. Computer or microprocessor 42 
is also provided which is connected via lead 44 to the base of inhibit 
transistor 36. A positive voltage source 40 is also connected to the base 
of inhibit transistor 36 through resistor 38. The collector of inhibit 
transistor 36 is connected via lead 34 to lead 32, which is connected to 
the base of drive transistor 24. 
In normal operation, when the pulse generator would be providing an output 
to enable meter 28 to operate as a tachometer or a speedometer, output 44 
of computer 42 would be a logic zero low impedance output thereby allowing 
voltage source 40 to, in effect, be grounded through resistor 38 and 
computer 42. Accordingly, the base of inhibit transistor 36 would not 
receive drive current and would be in an off or nonconducting condition. 
Outputs from pulse generator 20 would travel through resistor 22 and lead 
32 to the base of drive transistor 24. This would permit the intermittent 
turning on of drive transistor 24 and permit voltage source 30 to supply 
meter 28 with appropriate current to reflect correct meter operation as 
controlled by pulse generator 20 output. 
There are four conditions that are desired for the computer 42 to operate 
in a reset condition and thereby inhibit the meter operation. These 
conditions are an initial instrument power-up to eliminate erroneous meter 
needle jumps, low voltage supply to the computer and to pulse generator 20 
and malfunction of the computer under an internal or external reset 
condition. Whenever computer 42 is in such a reset condition, its output 
terminal would be in a high impedance condition. Accordingly, voltage 
source 40 would supply a drive current through resistor 38 to transistor 
36, thereby turning transistor 36 on or to a conducting condition. If such 
transistor 36 were in a conducting condition, output pulses from pulse 
generator 20 would travel through resistor 22 and lead 34 to ground 
through transistor 36. Accordingly, drive transistor 24 would not be 
turned on and thereby meter 28 would remain undeflected. After a 
preselected delay, computer 42 would change to a logic zero low impedance 
output and normal meter operation would occur.