An apparatus for determining a power frequency of a light source, the apparatus comprising: a photo-sensitive bipolar junction transistor, BJT, coupled to generate an electrical signal responsive to light incident thereon; a modulator circuit coupled to the BJT to generate a modulated signal based on the electrical signal that toggles at a rate substantially proportional to the power frequency of the light source; and a logic unit coupled to the modulator circuit to determine a toggling frequency of the modulated signal and to generate a frequency select signal indicating the power frequency of the light source from among at least two different potential power frequencies, wherein the modulator circuit comprises: a comparator including first and second comparator inputs and a comparator output, the first comparator input coupled to the BJT and the second comparator input coupled to receive a reference signal; and a charging source configured to selectively charge the first comparator input based on feedback from the comparator output; a switch coupled between the charging source and a circuit node that couples to the BJT and the first comparator input; a latch or flip flop coupled between the comparator output and the logic unit, the latch or flip flop comprising a sample input coupled to the comparator output; a sample output coupled to the logic unit; and a clock input coupled to receive a clock input; wherein a control terminal of the switch is coupled to the sample output of the latch or flip-flop to selectively control coupling of the charging source to the BJT and to the first comparator input responsive to the sample output; wherein, in response to the first comparator input being less than the second comparator input, the feedback from the comparator output causes the charging source to charge the first comparator input; and wherein, in response to the first comparator input being greater than the second comparator input, the feedback from the comparator output causes the charging source to stop charging the first comparator input. a photo-sensitive bipolar junction transistor, BJT, coupled to generate an electrical signal responsive to light incident thereon; a modulator circuit coupled to the BJT to generate a modulated signal based on the electrical signal that toggles at a rate substantially proportional to the power frequency of the light source; and a logic unit coupled to the modulator circuit to determine a toggling frequency of the modulated signal and to generate a frequency select signal indicating the power frequency of the light source from among at least two different potential power frequencies, wherein the modulator circuit comprises: a comparator including first and second comparator inputs and a comparator output, the first comparator input coupled to the BJT and the second comparator input coupled to receive a reference signal; and a charging source configured to selectively charge the first comparator input based on feedback from the comparator output; a comparator including first and second comparator inputs and a comparator output, the first comparator input coupled to the BJT and the second comparator input coupled to receive a reference signal; and a charging source configured to selectively charge the first comparator input based on feedback from the comparator output; a switch coupled between the charging source and a circuit node that couples to the BJT and the first comparator input; a latch or flip flop coupled between the comparator output and the logic unit, the latch or flip flop comprising a sample input coupled to the comparator output; a sample output coupled to the logic unit; and a clock input coupled to receive a clock input; wherein a control terminal of the switch is coupled to the sample output of the latch or flip-flop to selectively control coupling of the charging source to the BJT and to the first comparator input responsive to the sample output; wherein, in response to the first comparator input being less than the second comparator input, the feedback from the comparator output causes the charging source to charge the first comparator input; and wherein, in response to the first comparator input being greater than the second comparator input, the feedback from the comparator output causes the charging source to stop charging the first comparator input.