A print material level sensor includes a series of print material level sensing devices disposed at intervals to detect the presence of print material at successive depth zones in a container. Each print material level sensing device includes a heater to emit heat at its depth zone and a sensor to sense heat at the depth zone and to output a signal based on the heat sensed. The print material level sensor includes control circuitry to enable supply of electrical power to the heater of any one of the print material level sensing devices in its depth zone and to receive the signal from the respective sensor. The control circuitry includes a comparator to compare a value of the signal to a target value. The control circuitry disables supply of the electrical power to the heater when the value of the signal is at least equal to the target value.