An apparatus includes a syringe housing and a plunger slidably received within the syringe housing between a first position and a second position. The plunger has a substantially opaque portion and a substantially translucent portion. A light sensor module is disposed on the syringe housing. The light sensor module has a light sensor housing and a first sensor element and a second sensor element. The first sensor element and the second sensor element are disposed within the sensor housing. Positioning the plunger at the first position aligns the substantially translucent portion with the first sensor element and positioning the plunger at the second position aligns the substantially translucent portion with the second sensor element.
