Abstract: A transmitter circuit adapted to be connected to a sensor and generate a 4-20 milliampere signal proportional to the output of the sensor includes a first amplifier which is connected to the sensor and has an output voltage proportional to the sensor output voltage. A predetermined reference voltage is supplied to a reference input of the first amplifier. A second amplifier housing an output voltage responsive to a difference between its input voltages, has its output connected to the gate of an output transistor, preferably a fluid offset transistor. An output resistor is connected between the positive power supply and the source of the output transistor. The output of the first amplifier is supplied to the positive input of the second amplifier and the voltage at the source of the output transistor is supplied to the negative input of the second amplifier. The output current for the transmitter is generated at the drain of the output transistor.

Abstract: A trimode gas detection instrument having three operating modes, one which monitors the level of combustible gases, a second which monitors oxygen, and a third which monitors the displacement of air by an unknown gas. Only two sensors are used, a combustible gas sensor and an oxygen sensor. A switch selects the input to a readout so that the user can quickly observe the concentration readings in any of the three modes. In the depletion mode the readout is calibrated in relation to the inverse of the normal concentration of oxygen in air--i.e., zero depletion corresponds to an oxygen concentration of 21%. In the event that an unknown gas displaces air in a sample atmosphere, concentration of the unknown gas appears on the readout, such that zero oxygen corresponds to a concentration of 100% unknown gas.

Abstract: A bias circuit (13) for an electrochemical fuel cell (15) serving for example as a carbon monoxide or hydrogen sulfide detector, including an operational amplifier tied to the anode and reference terminals (21,23) of the fuel cell (15), and a zener diode (50) arrangement for maintaining a predetermined voltage drop between the output and negative inputs of the operational amplifier (33).

Abstract: A receiver and decoder is disclosed wherein the receiver may be used as a remote control receiver in conjunction with a transmitter transmitting a carrier modulated with an encoded data train of first and second modulation frequencies for first and second modulation patterns. A third modulation pattern may be established by providing successive portions of the first modulation signal and then the second modulation signal during a given time. If the receiver is on the same carrier frequency and is set to receive the same modulation pattern sequence as in the encoded data train from the transmitter, then the receiver has an output which may be used to control some remote load. This may be a garage door operator to open a garage door, for example.The decoder in the receiver is responsive to the modulation signals and a signal discriminator discriminates among the three different modulation patterns or logic conditions of bits.